[ACLs] Add custom text and link to sign up form

[gino-m]

trafficstars

For now we can add android:autoLink="web" to the dialog permission_denied_dialog.xml and customize the contact_survey_organizer_to_obtain_access string in the of-ground fork. In the future we should read the url from Firestore config/properties/signUpUrl @rfontanarosa FYI

See also https://github.com/google/ground-platform/issues/1679