[83] Create refresh token endpoint

[jacekkalbarczyk]

User login (auth/login/) endpoint is now returning both access and refresh token. Refresh token can be used to obtain new access token(via /auth/refresh/ route). There is a new endpoint (/auth/revoke-refresh-token/) to revoke refresh tokens (it should occur during logging out). I also added missing ending slashes to existing routes.
Use Postman app for tests.

Endpoints:

Login
(method: POST) /auth/login/
takes raw JSON as a parameter:
{
"username": username,
"password": password
}
If successful, returns access token, refresh token, HTTP status CREATED (201),
otherwise returns message "Not authorized", HTTP status UNAUTHORIZED (401).

Refreshing access token
(method: POST) /auth/refresh/
takes refresh token via Authorization header(Bearer token) as a parameter.

If successful, returns new access token, HTTP status CREATED (201),
If refresh token was revoked returns message "token has been revoked", HTTP status UNAUTHORIZED (401).

Revoking refresh token
(method: DELETE) /auth/revoke-refresh-token/
takes refresh token via Authorization header(Bearer token) as a parameter.

If successful, returns message "Refresh token successfully revoked", HTTP status OK (200).

[stanislawK]

I believe we could just use here: return {'access_token': access_token}, HTTPStatus.CREATED