Consolidate sending backoff for different types of sender.
Add MAPIT_ID_WHITELIST configuration option.

This allows someone to restrict FixMyStreet's usage to specific MapIt
IDs rather than a whole MapIt type. This is currently most useful
with Global MapIt as you probably don't want e.g. all the O04s in the
world for your installation.
Have --no-wrap for gettext-merge.