R1:a90b16e83a33

Define available Herald rule repetition options in terms of "isSingleEventAdapter()"

Summary:
Depends on D18924. Ref T13048. Each adapter defines which repetition options ("every time", "only the first time") users may select for rules.

Currently, this is all explicit and hard-coded. However, every adapter really just implements this rule (except for some bugs, see below):

> You can pick "only the first time" if this adapter fires more than once on the same object.

Since we already have a…