Specifying wildcard characters which also happen to be regex
meta characters for `auto_approve_email_domains`, `allowed_email_domains`
and `blocked_email_domains` site settings currently breaks email
validation.
This change prevents these characters from being specified for these
site settings. It does this by switching the site setting type
from `list` to `host_list`. The `host_list` validator checks for these
characters.
In addition, this change also improves the site setting descriptions and
introduces a migration to fix existing records.
743d3ea4f3