Split .gitlab-ci.yml file into multiple files

Intent

The all in one .gitlab-ci.yml files was unwieldy to read or modify.

This MR splits it into one file per job to make things easier to read.

Also:

• Re-enables language-exists which looks to have been disabled during come Crowdin issues
• Removes the unit-test job as it just echoes are there tests?
• Fix some typos
• Adds a comment for each job/file to better explain what it does.

---

• This is meant for a hotfix
• This is meant for the next release (see milestone)
• This needs more reviewers than normal; there may be controversy or high complexity
• This intentionally introduces regressions that will be addressed later
• There is/will be documentation changes on the wiki
• Please do not send commits here without coordinating closely with the owner
• This is a Build System change

Steps to Test

1. CI passes