Improve the REST API consistentcy

[speza]

We could do with improving the REST API consistentcy.

While working on #246 I noticed that the REST API is pretty inconsistent, whether its not using PUT correctly with a named param for updates, or just the general route names in general.

For example:
GET /api/v1/users (for list)
POST /api/v1/user (for create) - should be POST /api/v1/users
or
PUT /api/v1/secrets/update should be PUT /api/v1/secret/:key

Would be nice to make this consistent and maybe write some docs on how these should be strucutred in the future. Its a nessecary change if we go with the RBAC work imo :)