Added author username tooltip in issuable header

## What does this MR do?

Adds the authors username as a tooltip to the header of issuables

## What are the relevant issue numbers?

Closes #13771 

## Screenshots (if relevant)

![Screen_Shot_2016-07-12_at_16.00.34](/uploads/9485150f1c9d4006384f18d08a07430f/Screen_Shot_2016-07-12_at_16.00.34.png)

See merge request !5209

Improves left static sidebar behaviour

List of changes:

* removed bottom 50px space that is no longer necessary with pin button at the top
* increased side padding so sidebar links are closer to counters
* use fixed weight pin icon so it correctly aligned with counters


cc @annabeldunstone @jschatz1 

See merge request !5251

Fix duplicated entry in changelog [ci skip]

## What does this MR do?

Removes duplicate changelog entries.

## What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/19822



See merge request !5252

List of changes:

* removed bottom 50px space that is no longer necessary with pin button at the top
* increased side padding so sidebar links are closer to counters
* use fixed weight pin icon so it correctly aligned with counters

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Add a UI guide note about number_with_delimiter



See merge request !5220

Include default callback URL (OAuth)

## What does this MR do?
Adds information on the OAuth callback URL

## Are there points in the code the reviewer needs to double check?
None

## Why was this MR needed?
This information was missing.

## What are the relevant issue numbers?
http://www.twitter.com/designingcode/status/753511397124866049

## Does this MR meet the acceptance criteria?

- [X ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)

See merge request !5246

Fix markdown rendering for label references

Fixes #14424, #19753, #19760

See merge request !5224