Allow U2F 2FA without TOTP (#11573)

This change enables the usage of U2F without being forced to enroll an TOTP authenticator.
The `/user/auth/u2f` has been changed to hide the "use TOTP instead" bar if TOTP is not enrolled.

Fixes #5410
Fixes #17495

models/fixtures/u2f_registration.yml

@@ -1,7 +1,7 @@
 -
   id: 1
   name: "U2F Key"
-  user_id: 1
+  user_id: 32
   counter: 0
   created_unix: 946684800
   updated_unix: 946684800

models/fixtures/user.yml

@@ -542,3 +542,19 @@
   avatar_email: user31@example.com
   num_repos: 0
   is_active: true
+
+-
+  id: 32
+  lower_name: user32
+  name: user32
+  full_name: User 32 (U2F test)
+  email: user32@example.com
+  passwd: 7d93daa0d1e6f2305cc8fa496847d61dc7320bb16262f9c55dd753480207234cdd96a93194e408341971742f4701772a025a # password
+  type: 0 # individual
+  salt: ZogKvWdyEx
+  is_admin: false
+  is_restricted: false
+  avatar: avatar32
+  avatar_email: user30@example.com
+  num_repos: 0
+  is_active: true