Refactor filter function backend, add ability to assign a filter function to a problem part and hint.
JS function to send a hint to multiple students.
Allow sending hints to students who match a filter function from the web UI.
Update todo list.
Merge branch 'master' of github.com:yoavfreund/Webwork_AdaptiveHints

Conflicts:
	src/adaptive-hint-clients/teacher_client/ng/js/problem_part.js
	src/adaptive-hint-clients/teacher_client/ng/partials/problem_part.html