Currently, we don't send any emails when updating the attention set.
This change adds email sendings by default to OWNER, when adding
users to the attention set or removing them from it, but only when
added/removed explictly. Explict removals are through the review
endpoint, or through the dedicated attention set endpoints.
All of the explict updates allow specifying AttentionSetInput,
and now it is also possible to specify notify and notify_details
to change the receivers of the attention set emails.
Change-Id: Ifca8e2dd5de92a608fa987ea894d6326d1e88135
f2b1b3368e