Repository Explorer should support the <context-param> approach for loading repositories

With the changes for MODE-2304, it seems that the only way the Repository Web-Explorer can discover repositories is via:

 InitialContext ic = new InitialContext();
 ModeShapeEngine engine = (ModeShapeEngine) ic.lookup("jcr");

This works fine OOTB for the WF subsystem (where this binding is done automatically behind the scenes) but it extremely limiting in all other cases because:
a) users need to run with a JNDI RW environment
b) they need to make the above binding themselves

We need to make sure the context-param discovery approach works as well, meaning that users should be able to add to their web.xml either:

    <context-param>
        <param-name>org.modeshape.jcr.URL</param-name>
        <param-value>jndi:jcr</param-value>
    </context-param>    

or

    <context-param>
        <param-name>org.modeshape.jcr.URL</param-name>
        <param-value>file:/repo-config.json</param-value>
    </context-param>    

We should also update the default web.xml from the modeshape-web-explorer-war module to contain both the above approaches but commented out & documented, so that it's obvious how one can use this configuration.
Since this most likely means using the ModeShapeJcrDeployer context listener, this should be part (by default) of the modeshape-explorer web-fragment.xml and we should make sure it works correctly even when none of the above context params are provided (i.e. it doesn't crash).