Improve text in Security settings (#28393)

- en-US: Rename "Scratch Tokens" to "single-use recovery keys".
  Longer, but clearer.
- Improve titles
- TOTP: Improve description
- TOTP: Inform user about Scratch Tokens to encourage TOTP usage
- WebAuthn: Add loss of access warning

templates/user/settings/security/twofa.tmpl

@@ -16,6 +16,8 @@
 		<button class="ui red button delete-button" data-modal-id="disable-twofa" data-type="form" data-form="#disable-form">{{ctx.Locale.Tr "settings.twofa_disable"}}</button>
 	</form>
 	{{else}}
+	{{/* The recovery tip is there as a means of encouraging a user to enroll */}}
+	<p>{{ctx.Locale.Tr "settings.twofa_recovery_tip"}}</p>
 	<p>{{ctx.Locale.Tr "settings.twofa_not_enrolled"}}</p>
 	<div class="inline field">
 		<a class="ui primary button" href="{{AppSubUrl}}/user/settings/security/two_factor/enroll">{{ctx.Locale.Tr "settings.twofa_enroll"}}</a>

templates/user/settings/security/webauthn.tmpl

@@ -1,6 +1,7 @@
 <h4 class="ui top attached header">{{ctx.Locale.Tr "settings.webauthn"}}</h4>
 <div class="ui attached segment">
 	<p>{{ctx.Locale.Tr "settings.webauthn_desc" | Str2html}}</p>
+	<p>{{ctx.Locale.Tr "settings.webauthn_key_loss_warning"}} {{ctx.Locale.Tr "settings.webauthn_alternative_tip"}}</p>
 	{{template "user/auth/webauthn_error" .}}
 	<div class="flex-list">
 		{{range .WebAuthnCredentials}}