- Currently, the output of the #all_dashboard_paths method of all
dashboard services is collected and cached by the Repository model.
Unfortunately, this cache cannot be deleted when an update is made to
the #all_dashboard_paths method.

- This MR changes the Repository model cache to only contain a list of
user-defined dashboard paths under .gitlab/dashboards. The
all_dashboard_paths methods do not do any other expensive operations
(other than fetching a list of dashboards under .gitlab/dashboards).

- This change should remove the need to delete the cache when a change
is made to the all_dashboard_paths method.