User authorization.

This adds user authorization to the user pages and fixes #29. This also replaces parts of https://github.com/mex/rent-it-client/commit/9fe675b914229ca77b3bcbbc5bc2c3757bc7c6c4 that are already merged into master due to a bad rebase.

Regular users can:
- View their own profile.
- Edit their own profile.
- View their own orders.
- Delete their profile.
- Can add a subscription to themselves.

Admin users can:
- Retrieve list of all users.
- Search for users.
- View any user.
- View any user's orders.
- Edit any user.
- Delete any user profile.
- Can add a subscription to any user.

CC: @mex