Clean up authentication

[Innders]

• Use RTK mutation instead of axios.
• Set axios and RTK headers inside useLogin mutation.
• Clean up naming convention for user slice.
• Create a helper hook for checking for user auth.

https://redux-toolkit.js.org/rtk-query/usage/examples#authentication

[martastain]

Interesting. This should be probably mentioned as well #373

Actually - I think the branch of that PR could be used to resolve this.