Add a script to assist with updating the icon font

To simplify the process of updating the icon font and make
sure that nothing is missed out, add a script which takes
the icon font archive generated by icomoon and updates
the relevant files in the repository.

T-90

h/static/styles/vendor/ICOMOON.md

@@ -17,9 +17,5 @@ add the relevant icons and then use the app's _Generate Font_ facility.
     `h-icon-<name>` class name.
  4. Ensure all icons in the 'h' set are selected, then go to the 'Generate Font' tab in icomoon
     and click the 'Download' button which appears _within_ the tab.
- 5. From the downloaded archive:
-  * Extract `fonts/h.woff` -> `./fonts/h.woff`
-  * Extract `style.css` -> `./icomoon.css`.
-  * Edit `icomoon.css` to keep only the _WOFF_ format font as that is [supported](http://caniuse.com/#feat=woff) by our target browsers (IE >= 10).
-    The WOFF URL needs to be an inlined data URL until #2571 is resolved.
- 6. Commit the updated `selection.json`, `fonts/h.woff` and `icomoon.css` files to the repository.
+ 5. Run `scripts/update-icon-font.py <icomoon zip archive>` to update the icon font
+ 6. Commit the updated files to the repository.

scripts/update-icon-font.py

+#!/usr/bin/env python
+
+import argparse
+import os
+
+from base64 import b64encode
+from zipfile import ZipFile
+
+def main():
+    parser = argparse.ArgumentParser('Update the icomoon icon font from the provided archive')
+    parser.add_argument('archive', help='Path to .zip file generated by icomoon')
+    args = parser.parse_args()
+
+    script_dir = os.path.dirname(os.path.abspath(__file__))
+    vendor_style_dir = script_dir + '/../h/static/styles/vendor'
+
+    icon_font_archive = ZipFile(args.archive)
+    icon_font_archive.extract('selection.json', vendor_style_dir + '/fonts')
+    icon_font_archive.extract('fonts/h.woff', vendor_style_dir)
+    css_input_file = icon_font_archive.open('style.css')
+
+    css_output_file = open(vendor_style_dir + '/icomoon.css', 'w')
+
+    for line in css_input_file:
+        if "format('woff')" in line:
+            # inline the WOFF format file
+            woff_content = icon_font_archive.open('fonts/h.woff').read()
+            woff_src_line = """
+    /* WARNING - the URL below is inlined
+     * because the CSS asset pipeline is not correctly rebasing
+     * URLs when concatenating files together.
+     *
+     * See issue #2571
+     */
+    src:url('data:application/font-woff;base64,%s') format('woff');
+"""
+            css_output_file.write(woff_src_line % b64encode(woff_content))
+        elif "url(" in line:
+            # skip non-WOFF format fonts
+            pass
+        else:
+            css_output_file.write(line)
+
+
+if __name__ == '__main__':
+    main()