When a board creator is removed from the only group given Editor permission, the user loses the 'Add gadget' and 'Edit layout' buttons on the dashboard.

Issue Summary

When a board creator is removed from the only group given Editor permission, the user loses the 'Add gadget' and 'Edit layout' buttons on the dashboard.
The user can still edit the dashboard by browsing to Dashboards -> Manage Dashboards, but this causes confusion to users who are not familiar the multiple ways a dashboard can be modified.

Steps to Reproduce

1. Login as a non-admin user and create a dashboard (notice the 'Add gadget' and 'Edit layout' buttons at the top of the dashboard)
2. Add the user to a group
3. Edit the board to provide Editor permissions to the group
4. Remove the user from the group
5. Login as the board creator user and browse to the dashboard (notice the 'Add gadget' and 'Edit layout' buttons are missing)
6. As the board creator user, browse to Dashboards -> Manage dashboards page -> click the cog to the right of the dashboard -> click Edit
7. Remove the group from the Editors section
8. Browse back to the dashboard and notice the 'Add gadget' and 'Edit layout' buttons return

Expected Results

A board creator should never lose the 'Add gadget' and 'Edit layout' buttons.

Actual Results

A board creator can lose access to the buttons until the unassociated group is removed.

Workaround

As the dashboard creator, browse to Dashboards -> Manage dashboards page -> click the cog to the right of the dashboard -> click Edit -> and...

1. remove all groups from the Viewers and Editors sections, OR
2. add the dashboard owner to the groups in these sections.