Add `FacilityGroup.description` field.

This is for #1966 - Add group description.

Add a migration, modify the form, and modify the templates to add the group description with the group name for the hover text of the group link like manage and coach.

GOTCHA: Although the changes are in the `FacilityGroup` model, please notice that it has overridden the `app_label` of it's `Meta` class to `securesync`.  So to create or run the migrations specify the `securesync` app NOT the `facility` app.  E.g. `python manage.py schemamigration securesync --auto`

We need to be very careful with adding/changing fields on SyncedModels that have previously been shipped, as it tends to break syncing and crypto-verification. @aronasorman and @cpauya, could you be sure to check in with me to plan a strategy whenever doing so? Thanks!

(I'll go back and add some minversion stuff to this one)