refactor: introduce Account and account methods to AccountService

This MR introduces the Account interface and changes the flow for adding access tokens to use this interface.

This MR is best-reviewed commit-by-commit:

1. extracting getUserForCredentialsOrFail into a separate module, we'll need this for migrating credentials to accounts
2. create Account and change the flow for adding tokens to use accounts
3. We are sanitizing the instanceUrl to not have the trailing slash. Since the token_input module is now responsible for constructing the account, we have to sanitize the URL in token_input

Related to #298 (closed)