Support Switches with page() helper

[stevejalim]

This changeset adds support for django-waffle switches to our page() view helper. Previously, one had to rewrite the route as regular Django view in order to use the switch to show/hide the route

Summary

• Add optional enabling_switch parameter to the page() helper function
• When specified, the page returns 404 if the waffle switch is disabled
• Allows feature-flagged pages to be easily controlled via waffle switches

Usage

page("new-feature/", "mozorg/new-feature.html", enabling_switch="new-feature")

Test plan

• Added unit tests for switch ON (renders), switch OFF (404), and no switch (renders normally)
• Manual verification: create a test page with enabling_switch, verify 404 when switch is OFF and 200 when ON

Fixes #16992

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.09%. Comparing base (a96ae66) to head (7a89ae0).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16993   +/-   ##
=======================================
  Coverage   81.08%   81.09%           
=======================================
  Files         157      157           
  Lines        8313     8317    +4     
=======================================
+ Hits         6741     6745    +4     
  Misses       1572     1572           

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