Adopt style metadata to manage style title

[giohappy]

The current approach(es) to manage and retrieve style titles, and consequently legends, is a grey area, where a mix of solutions are implemented in GeoNode and its client.
The root issue is that the declaration of the style must be done inside the style body itself, and for this reason it also differs between style formats (SLD, CSS, etc.).

For a CSS it can be defined with the @styleTitle attribute. For an SLD the usual approach is to rely on the UserStyle.Title element.
This approach has several drawbacks, last but not least the difficulty for the client to take control of the title while the style content is being edited.

Geoserver provides a very useful feature for styles: metadata. A style can have associated metadata, that currently can be managed through style REST API PUT requestsm and can be retrieved doing a style REST API GET request with the XML format.

An example of a custom title metadata attribute is shown here below:

<?xml version="1.0" encoding="UTF-8"?>
<style>
   <name>circle_7___0925f5c0-a696-11eb-80b1-65a870ee53bc</name>
   <workspace>
      <name>geonode</name>
   </workspace>
   <format>css</format>
   <languageVersion>
      <version>1.0.0</version>
   </languageVersion>
   <filename>circle_7___0925f5c0-a696-11eb-80b1-65a870ee53bc.css</filename>
   <metadata>
      <entry key="title">Circle 7</entry>
   </metadata>
   <dateCreated>2021-04-26 13:48:20.461 UTC</dateCreated>
   <dateModified>2021-04-26 13:48:46.502 UTC</dateModified>
</style>

Currently neither MapStore nor GeoNode rely on this capability, but this would make the management of title far more robust and indepdent from the style format.

To have this in place the followin steps are requred:

• extend Geoserver style RET API POST endpint to accept the metradata and the new style in one shot. Without this a PUT right after the first POST is required, which can causes an inconsistent state in case of errors during the PUT request. Make the creation of a style and its metadata an atomic operation geosolutions-it/geoserver#317
• make the Style Editor rely on this metadata for the management of the style title, and stop parsing the style title from the UserStyle.Title element (as it currently does). An issue already exists for this Add metadata to Style Editor to manage title and description geosolutions-it/MapStore2#3750
• implement the parsing of the title metadata (if available) inside geoserver_restconfig, and use this to populate the sld_title field. GeoNode already uses this field in case it's populated