Fix links in Update guide 5.1 to 6.0

Links have missing slashes and additionally point to `master` branch.
This MR prevents a 404 error (missing slash) and points to the correct branch (6-0-stable) for comparison of the config files.

/cc @sytse

Fix typos in CONTRIBUTING.md

This MR fixes two typos in [CONTRIBUTING.md](CONTRIBUTING.md).

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

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

Make houndci prefer single quotes.

Release template

See merge request !972

Release template

Consider moving stuff like the packages from the 22nd to the 21st

See merge request !971

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

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

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

Fix huge merge requests

* Refactored suppress logic for diffs
* Increase grit memory limit (should mix most of 502 errors when create MR)
* show first 100 files for huge diffs

Fixes #1424

See merge request !970

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

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

Show milestone status on group milestone show page

Closes #1430

See merge request !969

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

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

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

Branches with no parent cannot be merged

Solves `undefined method 'reduce' for false:FalseClass`

Fixes #1415

See merge request !967

Remove thread-unsafe Dir.chdir from Satellite#lock

See merge request !968

per request project rules caching

Enforce md style.

This is made possible by changes in gitlab-grit 2.6.10.

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

A Developer cannot manage milestones

Improve login page

* use max-width for brand logo to prevent stretch of small images
* improve default brand logo to be centred on any kind of screen
* hide logo for small screens
* hide text for xs screens
* reduce amount of text on sign-in page

Fixes #1423

See merge request !966

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

Redirect on signin

Fixes #1346

See merge request !965