Add permissions and update migrations for MwebEvents

Permission control added in models/ability replicating what was made in the
gem.

refs #1115
First version of a customized index for events

Not entirely finished yet. Uses the same style used in the index for spaces,
with a few adaptations.
Includes changes in the index for spaces: moved some styles to base classes
that are now used also in the index for events.

refs #1115