Split app_mode: TLX into individual feature flags

[fushar]

For Judgels to be more widely used beyond TLX, we need to split this app mode into individual flags. The flags need to be applicable both in server, and in web client. E.g. the "Courses" tab in the web client should show according to a new "course enabled" flag.

Furthermore, I think it is better if the web client reads the flags via an endpoint, not "baked" in judgels-client.js.

Some ideas:

Use case	Flag	Note
User registration	jophiel_userRegistration_enabled	Already exists, but please check
User password reset	jophiel_userResetPassword_enabled	Already exists, but please check
User rating	jophiel_userRating_enabled
Courses	jerahmeel_course_enabled
Problemsets	jerahmeel_problemSet_enabled