Clear activities dialog on user switch

Fixes: https://trac.openmicroscopy.org.uk/ome/ticket/12502
Test: 
- Do something which creates an Activity (e. g. run a script). Switch user. Make sure the Activity dialog has been closed. Open it and make sure it's empty. Alternative: Create another Activity, which will open the dialog, and make sure it only shows this Activity.
- Close and start Insight again; switch user (without doing anything which triggers the Activity dialog beforehand); Insight should not crash.

--no-rebase