closes #1666
#1281 post purge
load composer-mobile.tpl if in xs/sm environment, for https://github.com/NodeBB/NodeBB/issues/1657
upgrading topics take a while, so let the user know
closes https://github.com/NodeBB/NodeBB/issues/1631
#1281 topic purge
closes #1281

category purge, deletes all topics and posts in the category