Add usermeta 'bp_docs_terms'

This piece of meta hasn’t been being added, due to an always-false
conditional. Doc editing doesn’t take place in the ‘bp_is_user()’
context anymore, so the sanity check was failing.
Add "no tags" message

Give users appropriate message when the “tag” filter control doesn’t
work as expected.
Fix incorrect variable name.

See #379

Props @dcavins
Don't let BP toggle off is_page during theme compatibility

We need it to remain on, because we use comments_template()
Readme
Bump version numbers to 1.5.7
Update .pot
Merge branch '1.5.x' into 1.6.x