[Settings] Change SMS preference setting to SMS auth availability

This toggle value isn't currently consumed anywhere in our codebase:

Our new authentication flow relies on the last successful login preference, not the "Prefer login codes via SMS" setting.

We should change this toggle to something along the lines of "Allow signing in with SMS login codes?", change the column to be enabled by default, add a check for this setting in `Login#available_factors` (make it reference a new `sms_available?` method), and write a backfill script to enable it for all users retroactively.

Start at Login#available_factors and trace the existing SMS preference setting and its column. Define the new sms_available? check, update the toggle label and default, then add a backfill that enables it for all users. Done means SMS availability follows the setting and existing users are enabled retroactively.

Written by the indexing model from the issue text.