API: allow specifying enabled integrations by name

[cuu508]

Suggested in #376:

Another way of having less API calls is if you added a parameter to let you pass in the channels by name rather than by id (it would assume channel names are unique) - that way I could skip the extra API call to get the channel id from the name :)

A few decisions:

• what to do when API client uses a channel name that does not exist?
• what to do when API client uses a non-unique channel name?

Could be strict or lenient here. I'm leaning towards strict to avoid subtle configuration mistakes: whenever a channel name in client's payload does not have precisely one match in the database, return HTTP 400 and a descriptive error message.