Enhancement: Tags form improved

We've improved the tags form in various ways, including visual appearance as well as usability. Auto save, remove tags on backspace, and contextual helper (and more, see issues)

https://github.com/owncloud/web/pull/9525
https://github.com/owncloud/web/pull/9563
https://github.com/owncloud/web/issues/9363
https://github.com/owncloud/web/issues/9356
https://github.com/owncloud/web/issues/9360
https://github.com/owncloud/web/issues/9362
https://github.com/owncloud/web/issues/9416

