fix MFA crashing on user-settings page

what it says on the tin. MFA (two-factor auth/one-time password) was hidden on user-settings but was actually crashing with errors in console. this fixes it so that FE properly handles case where BE doesn't support it yet.

src/components/user_settings/mfa.js

@@ -7,6 +7,7 @@ import { mapState } from 'vuex'
 const Mfa = {
   data: () => ({
     settings: { // current settings of MFA
+      available: false,
       enabled: false,
       totp: false
     },
@@ -139,7 +140,9 @@ const Mfa = {
     // fetch settings from server
     async fetchSettings () {
       let result = await this.backendInteractor.fetchSettingsMFA()
+      if (result.error) return
       this.settings = result.settings
+      this.settings.available = true
       return result
     }
   },