Add page for security keys with stubbed data

This adds a new platform admin settings row, leading a page which
shows any existing keys and allows a new one to be registered. Until
the APIs for this are implemented, the user API client just returns
some stubbed data for manual testing.

This also includes a basic JavaScript module to do the main work of
registering a new authenticator, to be implemented in the next commits.

Some more minor notes:

- Setting the headings in the mapping_table is necessary to get the
horizontal rule along the top (to match the design).

- Setting caption to False in the mapping_table is necessary to stop
an extra margin appearing at the top.

app/templates/views/user-profile.html

@@ -45,6 +45,14 @@
         {{ edit_field('Change', url_for('.user_profile_password')) }}
       {% endcall %}
 
+      {% if current_user.platform_admin %}
+        {% call row(id='security-keys') %}
+          {{ text_field('Security keys') }}
+          {{ text_field(current_user.webauthn_credentials|length) }}
+          {{ edit_field('Change', url_for('.user_profile_security_keys')) }}
+        {% endcall %}
+      {% endif %}
+
       {% if current_user.platform_admin or session.get('disable_platform_admin_view') %}
         {% call row() %}
           {{ text_field('Use platform admin view') }}