Use enums for MongoDB query selectors and update operators

[taesungh]

Made a mistake in #429: the database query for checking if a user already applied should use $exists as a selector instead of "exists" as a key.

irvinehacks-site/apps/api/src/routers/user.py

Line 106 in dd68c1f

Collection.USERS, {"_id": user.uid, "roles": {"exists": True}}, ["roles"]

To help avoid a mistake like this in the future, we can define our own str-like Enum in mongodb_handler.py that has named references for the Query Selectors we use (including $in) to help enforce this as a pattern. This should also be done for the Update Operators we use, e.g. $set.

[taesungh]

Hot-fix for typo added in #483, would still be good to add this at some point.