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`

There's no text in here to translate (just a hyphen), so the gettext calls should be applied to the raw values (prior to being inserted into the format string), not to the resulting string.

However, as these are user-supplied values, we shouldn't be translating them at all (since presumably users already entered them in the desired language). Could you go back and remove these, @cpauya, and target to nalanda-rct2? Thanks!