This commit refactors the code for sharing forums,
so it can be used for sharing blogs as well.
It does not yet include code for responding to blog invitations.
This allows for swiping left/right to read other posts by using
a ViewPager.
This hasn't been done as a separate activity, but with
fragments, so both can share the `BlogPersistentData` without
needing to reload it.
Closes#428