Review security rules

[generic-github-user]

Reading and writing rules, as well as incoming data validation.

For example, checking that ratings being sent to the database are in the 0 to 100 range, as they should be, and that the user has not artificially submitted a score input of 100,000.

[generic-github-user]

This should hopefully become somewhat easier as more processes are moved to cloud functions, instead of being handled on the client end: #39

[generic-github-user]

Reviewed Firebase Realtime Database security rules for creating and modifying post data. These are not included in the GitHub repository.