Add a "deprecated" property to extensions and hide deprecated extensions from the Extension Manager

Context: Extensions that are unmaintained, superseded by another extension or incompatible with supported XWiki versions (e.g. XEclipse, XWiki Workspaces, Balsamiq) are currently listed on extensions.xwiki.org (EXO) together with active extensions. Some display a manual warning box, others do not. They are also still listed in the Extension Manager (EM), where admins can install them by accident. (See full proposal: https://forum.xwiki.org/t/strategy-for-handling-deprecated-extensions-on-exo/18658/5).

Proposed solution:

• Add a boolean property deprecated (default false) to {ExtensionCode.ExtensionClass}}.
• When deprecated is true, EXO automatically displays a warning banner on the extension page, e.g.: "This extension is deprecated and is no longer maintained. It is kept for historical/reference purposes and is not recommended for new installations."
• Extend the listener that computes validExtension ("Installable with Extension Manager") on save, so that an extension marked as deprecated is marked as invalid. Invalid extensions are not listed in the REST API, so they no longer appear in the Extension Manager.
• Keep the extension page itself accessible on EXO.