feat: added ability to list, add, and delete users assigned fismasystems

[talentedmrjones]

This introduces the assignedfismasystems as a distinct resource apart from fismasystems

The following routes and functionality are included:

GET /users/{userid}/assignedfismasystems returns a list of fismasystem ids
POST /users/{userid}/assignedfismasystems creates a record of the assignment using the following body

{"fismasystemid": <int>}

DELETE /users/{userid}/assignedfismasystems/{fismasystemid} deletes the assignment

closes #108

[ATNoblis]

BIG Changes... took me a hot minute but LGTM