Milestone dropdown does not stay selected

Closes #23713

See merge request !7117

Resolve "Improve build "Retry" button on Build Page"

* On desktop, adds retry button on build page header
* On mobile, adds retry button in sidebar

![Screen_Shot_2016-10-28_at_1.35.07_PM](/uploads/2f9f87a9fc3e85c9aafc8c3d7a231639/Screen_Shot_2016-10-28_at_1.35.07_PM.png)
![Screen_Shot_2016-10-26_at_1.01.44_PM](/uploads/d02ce3a422a505b14ab357f695de8bba/Screen_Shot_2016-10-26_at_1.01.44_PM.png)

Closes #23273

See merge request !7126

Adds label description to issue board title

## What does this MR do?

Adds the label description as a tooltip the issue board title. Tooltip is not visible on the backlog or done list.

## Screenshots (if relevant)

![tooltip-title](/uploads/e807a7ff6c48ac2181c4d4e86adf0f77/tooltip-title.gif)

## What are the relevant issue numbers?

Closes #22710

See merge request !6743

Hide project activity tabs when features are disabled

## What does this MR do?

When features are disabled in project settings, the activity tabs are now correctly hidden.

## Screenshots (if relevant)

![Screen_Shot_2016-10-21_at_12.15.27](/uploads/3d85c027aa05487001c8abd624a99be0/Screen_Shot_2016-10-21_at_12.15.27.png)

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/19734#note_17224181

See merge request !7038

Add validation errors to Merge Request form

Closes #22448

See merge request !7163

New todos blank state

## What does this MR do?

Updates the blank state on todos.

### Blank state when user has previously had todos

![Screen_Shot_2016-10-12_at_12.40.35](/uploads/17d9a5a409d12bce73147e3879d9d62d/Screen_Shot_2016-10-12_at_12.40.35.png)

### Fully empty blank state

![Screen_Shot_2016-10-12_at_12.40.40](/uploads/5c0824096ced9f03b3ea5f892b70b08b/Screen_Shot_2016-10-12_at_12.40.40.png)

## What are the relevant issue numbers?

Closes #20833

See merge request !6823

If source branch or target branch doesn't exist in the project, display
form validation errors. Previously, this caused a 500 error code.

Refers to #23806.

Closes #22710