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>

web/src/css_variables.js

@@ -0,0 +1,39 @@
+/* eslint-env commonjs */
+
+"use strict";
+
+// Media query breakpoints according to Bootstrap 4.5
+const xs = 0;
+const sm = 576;
+const md = 768;
+const lg = 992;
+const xl = 1200;
+
+// Breakpoints for mobile devices used by Google Chrome as of Version 86
+const ml = 425; // Mobile large
+const mm = 375; // Mobile medium
+const ms = 320; // Mobile small
+
+module.exports = {
+    media_breakpoints: {
+        xs_min: xs + "px",
+        sm_min: sm + "px",
+        md_min: md + "px",
+        lg_min: lg + "px",
+        xl_min: xl + "px",
+        ml_min: ml + "px",
+        mm_min: mm + "px",
+        ms_min: ms + "px",
+    },
+
+    media_breakpoints_num: {
+        xs,
+        sm,
+        md,
+        lg,
+        xl,
+        ml,
+        mm,
+        ms,
+    },
+};