Tweak prompt on communities browser

Hopefully this makes it a little clearer how to add a community from the
browse page.
Use the usual nav place for join/leave

We had join/leave buttons for communities down in the #page area. This
puts them up in the box nav where the tip buttons usually go. This also
means that these elements are shown for viable communities; they weren't
before.
Bump version to 8.2.18
Bump version to 8.2.18-dev