i18n branches and/or repository should run full CI workflow

[cfm]

Description

In #6873 (specifically), we discovered the possibility of rare but obstructive partitions between the tooling that pushes to securedrop-i18n and the tests that run in securedrop. From the smallest to the largest possible solution to this problem:

1. securedrop@i18n-* branches should run the full CI workflow in addition to the translation test suite.
2. The securedrop-i18n repository itself should run full CI, including the translation test suite.
3. After freedomofpress/securedrop-engineering#30, pull requests from Weblate into securedrop will run the full CI workflow like any other.

User Stories

As a maintainer, I want as few surprises as possible, especially on release day.

[legoktm]

I think we should adopt 1) as a general policy - any commit slated to be merged into develop or a release branch must pass the full CI suite. I'm not really a fan of having special branch names trigger special jobs, ideally all tests should run always - are the translation tests too slow to run for every PR?

And of course we should do 3) too. :)