Add support for extensions categories

The idea is to introduce a way to ask a repository for flavors. Flavors being top extension used as entry point for the UI of an entire wiki.

• indicate that an extension is a flavor in pom.xml
• indicate that an extension is a flavor in ER
• add REST API to find flavors
• add EM API to search flavors in an ExtensionRepository
• allow listing an explicit list of flavors (and not search for them in that case)