Add a troubleshooting guide for SSL

See merge request !1900

If no artifacts are found, raise instead of printing a warning

See merge request !1891

Update unicorn worker to use 400MB as the variable in the per process calculation

Closes #2715

See merge request !1894

Fix mattermost log location

Closes #2718

See merge request !1881

Remove links to non-related issues

See merge request !1895

Fix paths to omnibus-mirror group

See merge request !1889

Adds support for setting env variables for the Mattermost runtime

Closes #2521

See merge request !1677

Unicorn workers currently are set to be reaped between 400 and 650MB.
Which were bumped up from 350MB a while back, but our caclulation wasn't
updated

Update the rounding fix in the numerator of the caclulation, which
hasn't been updated in a long time. And improve the comment so it isn't
missed in the future.

Reserved space has been bumped from 1GB to 1.5GB. This is 512MB for the
OS (the ubuntu minimum when not running Unity) and 1GB for the rest of
GitLab.

For determining what the rest of gitlab required, I ran `docker run
gitlab/gitlab-ee:nightly` then execed in and turned off unicorn using
`gitlab-ctl stop unicorn`

Then I ran `docker stats` to see the memory usage of the processes
running in the container. These were sitting just above 900MB.

Include Code of conduct to CONTRIBUTING doc.

See merge request !1892

doc: update: fix numbered list identation

See merge request !1885

Without proper identation, the list gets rendered as a bunch of separate
lists with just one item each, all numbered 1.

Mattermost 3.8+ automatically appends mattermost.log to the location.

Closes #2718

Use a trailing backslash because Mattermost in FileLocation

Update the artifact expire_in time to 14days for tags

Closes #2717

See merge request !1880

14 days matches what we have done for the staging uploads

During the 9.5.1 release, it appeared that the artifacts were removed
after a day, and we were no longer able to properly release.

Load omnibus.rb file during AWS task

See merge request !1866

Remove Helm chart docs from Omnibus

Closes #2700

See merge request !1863

Include a list of other places where the Git version should be updated

See merge request !1831

Resolve "Allow setting of arbitrary settings per pgbouncer database entry"

Closes #2628

See merge request !1858