[IMP] calendar: generic improvements

We need the calendar view to be consistent in the usage of time formats
currently the week column (on the far left) is not consistent with what
we display in the meeting time slot.

It is currently not ovious that you can add and view anybody's calendar
so we need to add a placeholder.

In order to create a meeting you have to click on create which is not
explicit.

Task #1838108
Closes #24675