Repositories displaying an "Add more..." tab on the header is a neat way
to let people discover they can enable more units. However, displaying
it all the time for repository owners, even when they deliberately do
not want to enable more units gets noisy very fast.
As such, this patch introduces a new setting which lets people disable
this hint under the appearance settings.
Fixes #2378.
Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>
