Fix sfu misc plugin

This does two things:

1. It moves the aaa_sfu_misc plugin out of vendor/plugins and into lib
2. It fixes how `@context` is handled in `block_student_access`. The old way, where it was set to `nil` at the end of the run, caused issues when bringing in the new analytics gem. 

I want to bring this into edge first; I've tested there manually but want to run a build and deploy before merging into sfu-develop.