So it appears GitHub already supports multiple types of subscriptions:
repositories and threads.  I'm leaving the data type simply as
'Subscription', but am updating the method names to avoid confusion,
since they're both part of the ActivitiesService.

I changed my mind.  This really is terribly verbose, and looking at the
design of the API, it seems unlikely GitHub would change the
subscriptions endpoint to include anything other that repositories.  And
if they do...  well, we'll deal with it.

this is a breaking change, but there's no way around that, since the opt
param was left off initially.

(I kinda hate how long some of these names are... I'd much rather just
call them "subscriptions", but the API docs is pretty consistent about
calling them "repository subscriptions", so that's what we go with.
Maybe the intend to later add other kinds of subscriptions?)