[web] Change apps tab to be a modal
The Apps tab is changed to be a modal. Mind that the designs have changed a bit, see comments under the issue.
Run web app, pressed the "+" button in the top bar, checked that a modal appears. Check that the modal displays correclty and it is possible to add / remove calendar as an app (the modal works
Reviewers: kamil, kuba, michal
Reviewed By: kamil
Subscribers: ashoat, tomek, atul
Differential Revision: https://phab.comm.dev/D6812