New endpoint for instant search

Part of #798 (closed)

What does this MR do?

Refactors some parts of the instant search in the topbar:

• Uses a DTO for search results in the SearchGateway
• Creates a new endpoint GET /search/all for the instant search in the topbar
• Removes the client-side wrapper code that mapped the legacy results from the Xhr endpoint
• Adds a permissions class for searches
• Makes all affected functions a bit more PHP 7.4 compatible

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

Quite sure, seems to work on localhost. The vue front-end already used the new format and doesn't change. The database request and overall behavior shouldn't change as well.

How to test

Steps a reviewer can take to verify that this MR does what it says it does e.g.

1. Checkout branch locally
2. Login as foodsaver
3. Search for a region, store, or another foodsaver in the topbar
4. Click the results and see if they lead to the correct pages

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))
• Once your MR has been merged, you are responsible to update the #foodsharing-beta Slack channel about what has been changed here. They will test your work in different browsers, roles or other settings

CHANGELOG.md

@@ -37,6 +37,7 @@
 - Wrap long email address in user profile #828 !1541 @ChrisOelmueller
 
 ## Refactoring
+- Make the instant search in the topbar use a new rest endpoint without legacy wrapping code for search results !1522 @alex.simm
 
 ## Dev/Test/CI stuff
 - Migrate gitlab CI config to use rules instead of only/except !1529 @NerdyProjects