Add calendar limit

[scrungus]

This probably needs a sanity check as it was rushed but seems to work.

This PR:

• adds a config section for quota settings
• enforces quota limits if enabled, where the quota for a period (default is 'week') is defined as quota_period * daily_average * usage_limit.
• adds unit tests which cover the following cases:

PASS:

• 'front loaded' prexisting light reservation
• 'back loaded' prexisting light reservation
  FAIL:
• 'front loaded' prexisting heavy reservation
• 'back loaded' prexisting heavy reservation

front and back loaded refers to whether the reservation starts before the quota period or finishes afterwards. we test this because we want to make sure we are only counting the proportion of the reservation within the current quota period towards the quota limit.

light vs heavy reservation refers to whether the reservation is small or large enough that it either prevents or allows a new reservation.