Remove OVERMIND_HEALTH.
“Under attack“ and “destroyed” messages from reactor and overmind.

“Under attack” messages occur when health dips below 75% and 25% unless the
same message was broadcast within the last 7½ seconds.
Restrict reactor & overmind messages to the target team.
Fix up EV_* text for new events.
Overmind, reactor & repeaters: warn about destroyed buildings within range.

The warnings are rate-limited; for any given area, they won't occur more
than once every 15s.

For aliens, buildings regarded as inside the base will be warned about as
the base being under attack.

For humans, buildings within the reactor's power radius or regarded as
inside the base will be reported as the base being under attack. Buildings
withing repeater power range will be warned about via the nearest repeater;
warnings are limited on a per-area basis (using area names) if possible,
else per-repeater (which could become spammy).

The defence computer is no longer used for ‘base under attack’ warnings.
Implement a fake location entity; use it to avoid ‘under attack’ spam.
Adjust distance calculations for under-attack warnings.
Switch back to G_InsideBase() & fix location checking.
Special-case repeaters to avoid possible erroneous “base under attack”.
Don't do PVS check if the building is definitely not inside the base.
Don't warn when (for example) moving reactor or overmind.
Merge branch 'master' into gameplay/ds
Merge branch 'gameplay/ds' into gameplay/testing