Investigate the possibility to use MapStoreExtension to extend the list of plugins available

[allyoucanmap]

It would be nice to support a mechanism to extend the available plugins to be used in the various application contexts (eg map, layer) without fork or branch the repo. There was some initial test on master to use the MapStoreExtension and we need to control if this process can support the requirement to extent the list of plugins.
If this process is working we should document it in the README file.

Initial Test

The MapStoreExtension should allow to create a custom plugins and configure them with a json file. This should be the expected structure in a geonode-project

geonode-project/
|-- ...
|-- project-name/
|    |-- ...
|    +-- templates/
|         |-- ...
|         +-- geonode-mapstore-client/
|              +-- _geonode_config.html
|    +-- static/
|         |-- ...
|         +-- mapstore/
|              +-- extensions/
|                   |-- SampleExtension/
|                   |    |-- assets/
|                   |    |-- translations/
|                   |    +-- index.js
|                   +-- index.json
|-- ...

where:

• geonode-project/project-name/static/mapstore/extensions/SampleExtension/ contain the unzipped compiled extension (sample downloade from the release)
• geonode-project/project-name/static/mapstore/extensions/index.json declare the extension to add in mapstore app:

{
    "SampleExtension": {
        "bundle": "/static/mapstore/extensions/SampleExtension/index.js",
        "translations": "/static/mapstore/extensions/SampleExtension/translations",
        "assets": "/static/mapstore/extensions/SampleExtension/assets"
    }
}

• geonode-project/project-name/templates/geonode-mapstore-client/_geonode_config.html modify the plugins configuration. For example the map_embed page

{% extends 'geonode-mapstore-client/_geonode_config.html' %}
{% block override_local_config %}
<script>
    window.__GEONODE_CONFIG__.overrideLocalConfig = function(localConfig, _) {
        localConfig.plugins.map_embed.push({ name: 'SampleExtension' });
        return localConfig;
    };
</script>
{% endblock %}

Now the /map/{pk}/emebed path should show the new plugin