UI: More consistent alignment and padding

See the commits for more details, the messages mostly speak for themselves.

# Highlights:

## Merge request form bottom

Before:

![mr_before](/uploads/de2ae608d300784b61818bc33538ac64/mr_before.png)

After: 

![mr_after](/uploads/9b1568d8287704ee9497cc029e827778/mr_after.png)

## Issue header

Before:

![issue_before](/uploads/e8cc4fb316219f025ff7bd74c7da8677/issue_before.png)

After:

![issue_after](/uploads/5fbad99f038571196c3c3c2bea304381/issue_after.png)

## Empty repository message

Before:

![empty_before](/uploads/f6ab80aea9af9111c3dc260d5fa98cd6/empty_before.png)

After:

![empty_after](/uploads/40a528f5a45f4e47776356c88aab24c4/empty_after.png)

Fixes #3742 and https://dev.gitlab.org/gitlab/gitlabhq/issues/2594

See merge request !1953

Also update gitlab-workhorse in patch updates [ci skip]

Fixes #3747 

See merge request !1959

Prevent impersonation if blocked

fixed the documentation of the Guest role in permission.md

This MR fixes the documentation of the Guest role.  

closes gitlab-org/gitlab-ce#3777 

cc @dblessing @JobV 

See merge request !1952

Make icon-link image transparent

[ci skip]

See merge request !1945

Update autocomplete_controller to be more readable

up for grabs label

cc @dzaporozhets @razer6 

See merge request !1938

Install gitlab-shell 2.6.8 in installations from source

[ci skip]

See merge request !1932

Update CI quickstart guide

This includes fixes suggested by @axil 

Fixes #3260 

See merge request !1755

Add ignore whitespace change option to commit view

* Remove references to enabling CI since it it will be deprecated
* Revert changes in yaml/README.md, refine it in a separate MR