Support XWiki backends older than 18.2 through a build-time flag

Problem

Since CRISTAL-795 and CRISTAL-812, the XWiki backend reads the edit right and the rendered content of a page from the checkRight and supportedSyntax parameters of the page REST resource. XWiki supports them only since 18.1.0 and 18.2.0. Against an older XWiki, such as the 17.10 LTS line:

• the Edit action is never offered, even to a logged-in user with edit right;
• pages that are not in Markdown, such as xwiki/2.1 pages, are displayed as raw source (for instance = Title = instead of a heading).

This is the case of https://cristal.demo.xwiki.com/, whose backend cannot be upgraded past 17.10.

Proposal

Add a build-time flag, VITE_CRISTAL_XWIKI_LEGACY_REST=true. When it is set and the page REST resource does not return the edit right or the rendered content, read them from the /rest/cristal/wikis/<wiki>/spaces/<space>/pages/<page>?format=jsonld endpoint of cristal-integration-rest-server (versions 1.4.0 and older; 1.0.0 to 1.3.0 support XWiki 16.x and 17.x).

The flag is off by default: the published packages and the default builds are not affected.