Clearly define resident login requirements

During our project kickoff, we identified login functionality as a critical point of potential complexity and agreed to define resident login requirements clearly.

There are two potential user stories here:

### As a user, I should be able to log into the RPP portal using my City of Austin utility account number so that I can conveniently access multiple City services with a single set of credentials.

_or_

### As a user, I should have the ability to log in to the RPP portal using my email address and a secure password.

(After discussing the option of third party credentials, #5 , we decided that solution isn't ideal.)

*Migrated from [atd-residential-parking-permits #3](https://github.com/cityofaustin/atd-residential-parking-permits/issues/3)*

Start with the two alternative resident login user stories in this issue and review the discussion referenced by #5. Resolve whether authentication should use a City of Austin utility account number or an email address and secure password. Done means one login approach is selected and its requirements are clearly documented.

Written by the indexing model from the issue text.