Add setting to select formula used for rent adjustment when ending tenancy mid-month

Currently, the formula used to calculate the default rent adjustment when ending a tenancy mid-month is:

• ((monthly rent x 12) / 365) x days in the pro rata period

This assumes an average of 30.417 days per month, and that a tenant is happy with a partial month's rent being calculated pro rata with respect to a year. It also doesn't account for leap years.

Therefore, I propose that an additional option is selectable in the settings that uses this formula:

• days in the pro rata period / days in the current month x monthly rent

So, for example, if a tenancy ends 8 days into the rental month, and the current month is March, then it would be 8/31 x monthly rent.