Add a styleguide page

There are now quite a few frontend components in the app.

It’s good to have a reference for them to:
- document for developers what patterns are available and how they are used
- check for regressions
  - when working on one variation of a pattern you can check that your changes
    don’t break other variations of it
  - when changing the arguments to a pattern you can check that this doesn’t
    change the expected arguments already in use

This commit adds a single page (`/_styleguide`) which has examples of all the
patterns.

app/main/__init__.py

@@ -4,5 +4,5 @@ main = Blueprint('main', __name__)
 
 from app.main.views import (
     index, sign_in, sign_out, register, two_factor, verify, sms, add_service,
-    code_not_received, jobs, dashboard, templates, service_settings, forgot_password, new_password
+    code_not_received, jobs, dashboard, templates, service_settings, forgot_password, new_password, styleguide
 )