web: Move web app to ‘web’ directory.

Ever since we started bundling the app with webpack, there’s been less
and less overlap between our ‘static’ directory (files belonging to
the frontend app) and Django’s interpretation of the ‘static’
directory (files served directly to the web).

Split the app out to its own ‘web’ directory outside of ‘static’, and
remove all the custom collectstatic --ignore rules.  This makes it
much clearer what’s actually being served to the web, and what’s being
bundled by webpack.  It also shrinks the release tarball by 3%.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

docs/development/using.md

@@ -75,8 +75,8 @@ the development environment][authentication-dev-server].
 - Most changes will take effect automatically. Details:
   - If you change CSS files, your changes will appear immediately via
     webpack hot module replacement.
-  - If you change JavaScript code (`static/js`) or Handlebars
-    templates (`static/templates`), the browser window will be
+  - If you change JavaScript code (`web/src`) or Handlebars
+    templates (`web/templates`), the browser window will be
     reloaded automatically.
   - For Jinja2 backend templates (`templates/*`), you'll need to reload
     the browser window to see your changes.