Web fixes

[Karinon]

This is a collection of several smaller fixes:

• fixed issue where the databrowser has shown an undefined-flavour when none was selected under certain circumstances
• replaced plain HTTPResponses by JSONResponses when the passed data was indeed a JSON
• added error message when react didn't load properly (prior to that: No info at all)
• added error message when there are issues with the user information (LDAP unreachable, Redis down) in one of the react-views (prior to that: infinite load animation)
• some smaller visual changes

[Karinon]

The failed CI-Checks are black-formatting issues and you have already fixed them in your own PR, therefore I will not touch it

[antarcticrainforest]

I can't really say much, most of the changes are beyond my knowledge. The only comment I have is about the HTTPResponse -> JSONRespons. There's more of them :)