Store user data using mobx

[larskendall]

User data is currently stored locally using the Principal service, and is updated in the database with the User resource. Instead, we should have a user store in mobx that uses actions to get and set user data. It can potentially still utilize the User resource... however, even having that class around may cause confusion and open the door to misuse outside of the store. We may want to consider removing the User resource and instead use axios in the store in order to force proper usage of mobx.