Change test-cluster plugin to misc plugin

Current uses:
- test cluster warning and student lockout
- adding hostname of app node that handled the request to the ENV var
  (JS global)

Fix file-encoding issue with test_cluster plugin

1. Loads the plugin properly to avoid the file-encoding issue.
2. Renames the plugin to aaa_sfu_misc to:
  a. resolve load-order issue (it must load before the recaptcha plugin)
  b. plugin can be used for other misc injections and stuff

move actual code to lib/test_cluster and require it in init.rb

rename plugin to aaa_sfu_misc

Plugin needs to load before the recaptcha plugin because Rails.

add APP_NODE to JS ENV var