initial_type refactoring

[jesteria]

The existing use of User.initial_type suggests some reconsideration.

• A value of DSSG_STAFF dictates whether a user can use the custom admin page, (to approve volunteer profiles – but nothing else)
• …While a value of ORGANIZATION dictates whether a user can perform some organizational functions
• Users are given DSSG_STAFF only when created by developer CLI
• …And either ORGANIZATION or VOLUNTEER during registration
• Users should likely be given the opportunity to perform these functions regardless of how they were created/registered.
• It is slightly odd that users have an "initial type" which dictates later permissions (and no further "type").

Documentation

Originally posted by @jesteria in #27 (comment)

[jesteria]

It seems possible that the app could do without initial_type entirely. This may be slightly inconvenient to the registration workflow, (but likely resolvable). User permissions and workflows following registration/creation should then be dependent upon other values.