Server: Add possibility to un-favorite file by clicking on the star

[MariusBluem]

*Nextcloud-version: 13.0

[MorrisJobke]

I would not do that ...

cc @nextcloud/designers

[jancborchardt]

Yes, I wouldn't do it either because:

• It will need to have 44*44 area of clickability which extends well beyond the icon which will lead to misclicks, especially on mobile.
• A toggle-action like this should be toggleable then, but then we need to introduce a grey star again - nope.