Reimplement store wall in REST

What does this MR do?

• Reimplement the comment part of store walls in Vue
• Reimplement writing, fetching and removing comments in REST
• Markdown for store wall comments
• highlight wallposts by store managers
• allow store managers to remove wallposts which are older than 3 weeks one month

Please check each commit individually when reviewing, I spent a lot of time to find + build them into reasonable chunks.

How confident are you it won't break things if deployed?

surprisingly confident, although right now there will be changes in behavior that won't sit well with some people.

Links to related issues

• Epics: &9 and &26 (closed) and &30 (closed)
• implements #92 (closed)
• relates to #982 (closed) since this could now be implemented in a similar dynamic screen
• relates to #921 (closed) but stores aren't using Wall yet
• closes #22 (closed)
• closes #672 (closed)

How to test

Anything that relates to writing and removing wallposts in stores:

• with different users, write posts, view and remove them
• in the database, make some of those posts older than 3 weeks one month (or use the seed data store with old notes)
• as store manager: be able to remove the old comments but not newer ones (unless they're your own, or you are Orga)

Screenshots (if applicable)

Checklist

• added a test, or explain why one is not needed/possible...
• update last handling for dashboard => !1752 (merged)
• no unrelated changes
• asked someone for a code review
• set a "for:" label to indicate who will be affected by this change
• use "state:" labels to track this MR's state until it was beta tested
• added an entry to CHANGELOG.md
• add a short text that can be used in the release notes
• Once your MR has been merged, you are responsible to create a beta-testing issue

Release notes text

Die Pinnwand in Betrieben wurde grundlegend erneuert. Sie zeigt nun ausschließlich Textbeiträge an, welche mit Markdown formatiert werden können. Pinnwand-Einträge von Betriebsverantwortlichen erhalten eine farbige Markierung. Außerdem können Betriebsverantwortliche nun alte Einträge entfernen. Die Grenze liegt zurzeit bei einem Alter von einem Monat.