Make the 'People online' block collapsable

[Kristina Hoeppner]

The suggestion is to make the 'People online' side block collapsable so that you can decide to show people or not (see attached screenshot). This is a useful option when you don't want to turn the block off entirely, but don't want to show names in that block during a training session or video recording.

A code suggestion is attached.

For implementation into core Mahara the following should be considered:
- Use our normal chevrons for collapsing instead of the two arrows in the picture.
- Could we use our normal card so as to get the chevrons directly?
- The collapsing should be sticky, i.e. remember the last state so one doesn't have to close it all the time when one just wants it closed for more than a page.

The attached code has some colour values hard-coded, which shouldn't be the case for Mahara core.