Fix drop meter display

* Do not calculate % of drop quota consumed when
the river has no drops
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Rename 'subscribed' property to 'following'

* Subscriptions and followers are not the same. Subscriptions relate
to services that receive content from SwiftRiver via push. The spec
for this feature is still in the draft stage
Follow/unfollow users

* Added the follow button on the user's profile page.  When viewing the
profile of another user, the client issues performs an API request to check
whether the currently logged in user is following the visited user.