Allow users to hide all "Add more units..." hints

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>

models/user/user.go

@@ -146,6 +146,7 @@ type User struct {
 	DiffViewStyle       string `xorm:"NOT NULL DEFAULT ''"`
 	Theme               string `xorm:"NOT NULL DEFAULT ''"`
 	KeepActivityPrivate bool   `xorm:"NOT NULL DEFAULT false"`
+	EnableRepoUnitHints bool   `xorm:"NOT NULL DEFAULT true"`
 }
 
 func init() {