This commit autoloads plugin files, and also extracts features into their own modules.
- `plugin.rb` is smaller
- external plugins like discourse-automation and discourse-assign have their own entrypoints
- solved filters as well
c.f. de983796e1b66aa2ab039a4fb6e32cec8a65a098
There will soon be additional login_required checks
for Guardian, and the intent of many checks by automated
systems is better fulfilled by using BasicUser, which
simulates a logged in TL0 forum user, rather than an
anon user.
