Make browse sources query customisable

This refers to the query run when using the **BROWSE SOURCES** button on the home page. It's currently hardcoded and it would be useful to allow mine admins to tweak it for their data model to display more useful results.

One approach would be to have a key in web.properties to define this query, and when present it will override the current default one.

Start at the home page implementation for the BROWSE SOURCES button and trace the query it runs. Then inspect web.properties and the existing default query; done means mine admins can provide an override while the current query remains the fallback.

Written by the indexing model from the issue text.