Resolve "No access through user_participation when creating group page"

This makes leaving a group or removing a user less cumbersome.

We should investigate the creation of user participations when editing a group page. But this should be a separate issue.

Incomplete list:

Page actions which create a user participation:

• starring
• watching
• sent notification (only if checkbox 'current access' is ticked)
• add attachment to page
• upload / remove picture (gallery page)
• edit (update) picture (file page)

The sidebar is reloaded after some of the actions (not for starring)

Page actions which don't:

• edit (wiki page)
• add comment
• make page public
• share page

Closes #216