Allow defining preferred time unit when syncing estimates from GitHub

It is possible to map estimate and work done fields to a GitHub project custom number field. This works well with story points, but if you are using time for your units, your values automatically convert to minutes.

When sending data from the Aha! side, this means a "1h" entry passes through as "60" to GitHub.

When sending data from the GitHub side, this means a "1" entry passes through as "1min" to Aha!

It is rare for minutes to be the entry value of choice - the GitHub side would more commonly track hours, days, or weeks, and there is no way to set that in the integration.

When integrating with a GitHub project that allows custom fields, add a setting that sets the time unit preferences for values entered in all estimate and work done fields. When numbers are received from GitHub, they should be in that unit.