auto notification about new release

In admin panel show notification that the new version of the software is available when new release was uploaded on webdav.

changed milestone to %v15.0.0~alpha.2

added enhancement label

created merge request !986 (merged) to address this issue

mentioned in merge request !986 (merged)

closed via merge request !986 (merged)

mentioned in commit df5cff44

changed milestone to %v15.0.0~alpha.0