Add extended support for linked resources

[giohappy]

Now that GeoNode/geonode#11565 landed in master, any type of resource can be linked with any other resource type. Multiple relationships can be created:

• automatically (as before) between maps and datasets
• manually, through the "Related resources" under the Optional metadata tab of the Metadata editor, links can be assigned between one or many resources (any type)

IMPORTANT: For the moment the direction of links is depicted with the <<< (linked from) and >>> (linked to) chars inside the "Lniked resources" tab of the information panel. This is only temporary since a new visualization is on its way.
The documentation will integrate screenshots and explanations of the new panel once it's ready.