Freeze all events in the store
Currently any event stored in the js-sdk can be modified by anything
that does event.getContent() and then, accidentally or otherwise,
modifies the object that is returned. This has scope for all sorts
of strange behaviour and is generally terrifying.
This PR freezes the events as they go into the model objects,
although the event class itself does quite a bit of manipulation
of this object (mostly in redactions) so there's quite a bit of
code making a copy, modifying it and the refreezing it.
This commit just modifies the events to be frozen and doesn't
attempt to fix up any of the places we've been modifying the
events, so this currently breaks everything.
TODO: the interactions between redactions and edits look somewhat
subtle: more may be needed here.
