Add the ability to customize index.html (#113)

User may provide a custom index.html template file to be used instead
of the built-in version. Changes to this file are watched by laminard
using inotify in order to load and compress the custom file for gzip
delivery, reusing the existing method for serving static assets.

This feature obviates the custom css feature, so remove references from
the manual and add a deprecation warning if it is used.

Add a section to the UserManual describing how to use this feature and
including a link to an example using Semantic UI.

UserManual.md

@@ -616,11 +616,11 @@ DESCRIPTION=Anything here will appear on the job page in the frontend <em>unesca
 
 Change `LAMINAR_TITLE` in `/etc/laminar.conf` to your preferred page title. Laminar must be restarted for this change to take effect.
 
-## Custom stylesheet
+## Custom HTML template
 
-If it exists, the file `/var/lib/laminar/custom/style.css` will be served by laminar and may be used to change the appearance of Laminar's WebUI.
+If it exists, the file `/var/lib/laminar/custom/index.html` will be served by laminar instead of the default markup that is bundled into the Laminar binary. This file can be used to change any aspect of Laminar's WebUI, for example adding menu links or adding a custom stylesheet. Any required assets will need to be served directly from your [HTTP reverse proxy](#Service-configuration) or other HTTP server.
 
-This directory is also a good place to add any extra assets needed for this customization, but note that in this case you will need to serve this directory directly from your [HTTP reverse proxy](#Service-configuration) (highly recommended).
+An example customization can be found at [cweagans/semantic-laminar-theme](https://github.com/cweagans/semantic-laminar-theme).
 
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