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