Weblate database switch to PostgreSQL

Originally created by @hefee on #16994 (Redmine)

Upstream only wants to support PostgreSQL and kick out MySQL support. (see https://github.com/WeblateOrg/weblate/issues/2825, https://docs.weblate.org/en/latest/admin/install.html#database-setup)

Related issues

• Related to tails#16995 (closed)
• Related to #16943 (closed)
• Related to tails#17050 (closed)
• Related to #17445 (closed)