Angular A/T editor: make granularity a combobox

Following up on bug 1855780 (the Angular A/T editor), there's a minor regression to deal with: the new editor no longer supplies a combobox for the event granularity.

The Dojo version of the interface included a combobox that supplied the following values:

hourly
daily
weekly
monthly
yearly

This should be added to the Angular version, but it's important to note a couple wrinkles:

- it should be a true combobox, not just a select, and permit the values listed above, the empty value, /and/ any arbitrary granularity value that the admin might want to set up
- alternatively, maybe it's time to add a new table to store defined granularities and add an interface to manage them (possibly as just part of the overall A/T editor)

Evergreen 3.9-beta