Set last activity when accepting answer

[DLu]

I believe this is a bug that is affecting my API endpoint.

• Here is an example question that I recently marked as having an accepted answer. Note that last_activity_at=1557194036
• Here is the accepted answer which was added_at=1557194036 (which is the same as the last activity)

This means that my action of accepting the answer is not reflected in the last_activity_at timestamp, which makes it difficult to keep track of which questions don't have accepted answers.

I believe this PR should fix it, but I'm not 100% certain and haven't tested it locally yet.

[DLu]

Thoughts, @martin-bts?

[martin-bts]

So far, I mostly concerned myself with how Askbot is implemented, not so much with what is implemented.

This one liner seems sensible to me, but is this really the only code change required to properly address the issue? I don't know. I can't tell. You will have to convince Evgeny.

[evgenyfadeev]

@DLu will have a look over the weekend, maybe you're right.

[DLu]

:+1: