Email help text on registration should be updated without binding

[jryans]

The email help text on registration says "Use an email address to recover your account. Other users can invite you to rooms using your contact details.". Since we have now stopped (#10424) binding on registration, it's no longer true that others can invite you by email by default, so we should change this text.

[jryans]

Marking this blocked on design to tweak the copy. (Alternately, we could just delete the offending sentence.)

[nadonomy]

Context (mostly a reminder for myself), we bumped the aggregated terms modal on sign up to phase II, and until then users consent to things contextually throughout the app.

Copy suggestion:

Set an email for account recovery. Use email or phone to optionally be discoverable by existing contacts.