Add default role to users table and make roles enum

[maxachis]

Currently, in the users table in the database, roles are either "admin" or null. This forces the middleware logic to make assumptions as to what kind of role a user has, typically defaulting to "user". When adding users, we should probably make sure their role is set to a default value of "user", rather than null.

Additionally, we may want to ensure the roles are limited only to a set of enums. Currently, the only enums would be "admin", "user", but this could conceivably expand in the future.

[maxachis]

Created a dev script update to this, but will need to complement with a web app update which is to be merged at the same time.

[josh-chamberlain]

I think we'd deprecate the roles concept when we close #162—we could have an admin boolean flag for users instead of roles.

[maxachis]

Josh is right! This issue can thus be closed.