Auth Gateway: Extract localStorage key to separate property

[paulstatezny]

Auth Gateway: Extract localStorage key to separate property so that it can be modified by children easily.

Acceptance Criteria

1. The key of the localStorage property where the auth request token is stored is a separate property.
2. The prefix for the Authorization header is a separate property.

   Tasks

   • Extract those properties.

     Additional Notes

   • This will help Lively not to have to re-implement as much code to make its generic client. (See synapsestudios/lively#111.)

[paulstatezny]

:zero: :arrow_up: :sunny:

[paulstatezny]

:star:

[zpchavez]

Do those literal strings appear anywhere else?

[paulstatezny]

@zpchavez Nope

[zpchavez]

:+1: