Draw routes at root

This means that mounting forem at /forums now will produce routes like /forums/forum-name-goes-here rather than /forums/forums/forum-name-goes-here

Fixes #489
Forum routes should be matched last so that they do not interfere with admin
Use POST for subscribing/unsubscribing from forum topics.

Fixes #522
Implement 'Nil User' pattern to deal with deleted users

Potential fix for #427
Use Workflow 1.0.0 and change update_attribute calls to update_column calls.

There's also been a change to use Workflow's hook methods, so that Post and Topic both have their own #approve methods now.
Add missing translation keys and lookups

Fixes #523