Add support for events

[donno]

It should be possible to create events, such that geometry can be
attributed to a certain event. The web interface and APIs could then allow
the user to filter things down to a certain event so they don't see things
from a previous hunt.

While not necessary for a server being hosted on a PC in a car during
an event, it would however be useful for a public hosted server.

Geometry created would have to be associated with an event or be chosen to
be "permanent", i.e like the locations of where radio relays are, which
would be shown regardless of the event.