adding support for django directives

Following PR https://github.com/openmicroscopy/openmicroscopy/pull/3020 Adding custom Django directives (see https://docs.djangoproject.com/en/1.6/internals/contributing/writing-documentation/):

doc glossary in ../sysadmins/config.html should contains nicely formatted OMERO.web properties. All of them can be referenced from another pages by ```:setting:`PROPERTY_NAME` ```

This PR can be tested independently:

create description units:

```
.. setting:: FORCE_SCRIPT_NAME 

FORCE_SCRIPT_NAME
-----------------

``bin/omero config set omero.web.force_script_name``

Default: ``None`` 

Description: Used as the value of the SCRIPT_NAME environment variable in any HTTP request.

```

create a reference link 

```
text :setting:`FORCE_SCRIPT_NAME` text
```

check if builds and links properly