Display link to associated store in store-team chats

Closes #1118 (closed)

What does this MR do?

Frontend and database code is working.
Waits for the StoreModel wrap-up to implement new functionality in the backend.

I'm really sorry about the load of unrelated UI changes, but it was impossible to ignore this steaming pile of ... old code ... while working on it.

• Prepare database for connecting conversations with a store (Phinx migration) for MR: sql migration beta
• Annotate Message\Conversation members in PHP (plus some fixes for that)
• Fetch store ID and display link when querying conversation details
• Allow storing storeId when creating a Conversation

Unrelated changes:

• Chat UI: Rescue a lot of ancient legacy JS code (msg.js, CSS)
• Chat UI: Rescue a lot of ancient legacy JS code (conv.js)

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

I'm feeling rather confident about them all in all. But of course it cannot be merged in the current state, it's more meant to serve as motivation to finally tackle StoreModel refactoring at this point ;-)

Links to related issues

Product team: Verlinkung des Betriebs bei Teamchat-Nachrichten ( Beta | Live )

How to test

Only by manipulating database entries right now:

• check out branch
• run phinx migration, e.g. by stopping + starting containers
• manually insert matching storeId for the team conversation of a store
• open team conversation chat (you should see a link to the store in its title now)

Screenshots

Checklist

• added a test, or explain why one is not needed/possible...
• 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 testing issue in Beta Testing Repo

Release notes text

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