Resolve "Web extension: Cannot advance past first step in registration"

Closes #147 (closed)

• Default to app.passit.io
• Use constant instead of string "app.passit.io"
• Use register on init to check and set the url. Error handling is not set up though if it would fail.

Tangent notes

• What is urlDisplayName? Can this be removed?
• I think when submitting the url, some error is shown very briefly... but then it succeeds.