Allow GFM autocomplete to be trigger without the preceding space

## What does this MR do?

Gives the ability to GFM autocomplete to be trigger even if there is no preceding space.

I've taken the regex from the at.js plugin & tweaked it to allow the leading character to be a special character.

## What are the relevant issue numbers?

Closes #19975 

## Screenshots (if relevant)

![Screen_Shot_2016-07-21_at_14.41.34](/uploads/19684ba286baeedb754e7457945480a8/Screen_Shot_2016-07-21_at_14.41.34.png)![Screen_Shot_2016-07-21_at_14.41.40](/uploads/a77349bce599ae93b4bcddd355087f5c/Screen_Shot_2016-07-21_at_14.41.40.png)![Screen_Shot_2016-07-21_at_14.41.46](/uploads/c35df17b678b24b73c94b181f0784188/Screen_Shot_2016-07-21_at_14.41.46.png)

See merge request !5395

Fix title case to sentence case in "Add Todo"

## What does this MR do?
Implements sentence case for `Add Todo`

## Are there points in the code the reviewer needs to double check?
I changed the images used in the file `doc/workflow/todos.md` to show the new look.

## Why was this MR needed?
#24748 

## Screenshots (if relevant)
Before
![Screen_Shot_2016-11-21_at_12.03.21_PM](/uploads/225abe2901aacccea697596853bdc52c/Screen_Shot_2016-11-21_at_12.03.21_PM.png)
![Screen_Shot_2016-11-21_at_11.58.17_AM](/uploads/3a5997eb6bb037f1c581d9925fa288ce/Screen_Shot_2016-11-21_at_11.58.17_AM.png)

After
![Screen_Shot_2016-11-21_at_12.03.38_PM](/uploads/3be4d58e5adaa10fab759f8726a4d257/Screen_Shot_2016-11-21_at_12.03.38_PM.png)
![Screen_Shot_2016-11-21_at_11.58.37_AM](/uploads/016cdb32cb1e87e9e3cfd89309b53a4a/Screen_Shot_2016-11-21_at_11.58.37_AM.png)


## Does this MR meet the acceptance criteria?

- [X] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [X] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [X] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [X] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?
#24748

See merge request !7724

Closes #24680

Issues empty state

## What does this MR do?

Adds the empty state for the project, dashboard and group issues.

## Are there points in the code the reviewer needs to double check?

## Why was this MR needed?

## Screenshots (if relevant)

### Filtered to show no issues (on group issues page in this case but also dashboard and projects)

![Screen_Shot_2016-11-08_at_20.55.56](/uploads/b598af4a8a5e2c9fbb859abf36e70e4b/Screen_Shot_2016-11-08_at_20.55.56.png)

### Project with no issues

![Screen_Shot_2016-11-08_at_20.57.08](/uploads/8f54fdf1b3101c46299249fa2944207d/Screen_Shot_2016-11-08_at_20.57.08.png)

![Screen_Shot_2016-11-08_at_20.57.17](/uploads/1d2d162e1d845dd05e945d8ebb1d2101/Screen_Shot_2016-11-08_at_20.57.17.png)

### Group with no projects with any issues

![Screen_Shot_2016-11-08_at_20.54.55](/uploads/f28dc2038839d5bda0eb37f37927d5db/Screen_Shot_2016-11-08_at_20.54.55.png)

## Does this MR meet the acceptance criteria?

- [ ] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?


Closes #20840 

Closes #20850

See merge request !7309

Added new spec step

Added empty state to dashboard

Split empty and filtered empty states

Moved empty_state icons into their own folder and DRY up empty state html

Fixed failing spec

Added to groups page

Review changes

When searching for the author in the assignee dropdown it now correctly returns the user

Closes #22905