Keywords lost on style updates

[etj]

When a style is edit for a dataset, the keywords in the dataset metadata are replaced with the keywords set in geoserver.

It has been reported that also other actions trigger the same behaviour (change the thumbnail, change layer proprties)
Probably the procedure that sync info from geoserver syncs too much.

To find out if the error happens in your instance:

• in geoserver add the keyword "keyword_from_geoserver" to the layer
• in geonode, edit metadata and add the keyword "keyword_from_geonode"
• in geonode, edit the style and save
• in geonode, open again the metadata editor and check the keyword has been replaced

This misbehaviour is happeneing on dev demo (5.1.0.dev0), but not on stable demo (5.0.0.dev0)

• We do not want keywords to be synch'ed on style change.
• It has also to be investigated if this sync is also triggered by something else.
• When keywords are imported from geoserver, they should not be replaced, but merged with the existing ones in geonode.

[etj]

May be related to this #13951

[giohappy]

@sijandh35 I suspect this is a side-effect of c63392c.
The update MUST NOT replace existing keywords, but update (merge) them.

I also suggest making the sync of GS keywords optional.