Track security issues for the translation platform

Originally created by @drebs on #17378 (Redmine)

The translation platform currently runs software that doesn’t come from Debian (Weblate + dependencies), and we currently have no way to track security issues for them.

Some ideas to deal with this are:

• Develop a way to automatically get notified and maintain and enforce a workflow to manually upgrade when needed.
• Invest time into packaging more Weblate dependencies and trust package maintainers to do a good job.
• Create a script that fetches versions from Github and checks for patches for the current running version (i.e. filter for major.minor and check if there are bigger versions available).
• Use an online API to check for CVEs for Weblate (example: https://www.circl.lu/services/cve-search).
• Other possibilities?

Constraints

• Notifications should be sent to the Translation Platform admins list.
• No special per-admin steps should be needed. (eg. setting up local e-mail filters)

Sources of important security info

• django-announce mailing list (RSS).
• Weblate releases RSS feed.

Proposed steps

• Setup a way to get RSS notifications in administrative e-mail.
• Subscribe the above sources to send e-mail to Translation Platform admins list.
• Implement case insensitive post-process security filter for rss2email subscriptions.
• Subscribe to get release updates for all Python dependencies installed from upstream repositories.