Add automatic geogit root AuthorityURL and per layer authority identifier.

Whenever a geogit layer is configured, a catalog listener will check
that a "root layer" authority URL exists by the name of GEOGIT_ENTRY_POINT,
and creates a LayerInfo's LayerIdentifierInfo for that authority name
with the identifier <workspace name>:<store name> to locate the corresponding
layer's entry point under the /geoserver/geogit/<workspace name>:<store name>.

The catalog listener also takes care of updating the layer infos identifiers
whenever a workspace or datastore info is renamed.
Make geogit's layer authority identifiers also report the store's branch or head.

The format for the layer identifier is <ws name>:<store name>[:<branch/head>]
where <branch/head> corresponds to the gegoit datastore BRANCH or HEAD
connection parameter, whichever is present.