cubicweb-mercurial-server #2459813 Create entities class to move functionalities from a hook mixin [done]

Functionalities in the MercurialServerAdminMixin in the does not belong here (any longer), because there will be several mercurial-servers represented as entities instead of one configured in the instance config file.

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closed by#c290c0d362ed [hooks, entities] closes #2459813 : Move mercurial-server behaviour from hooks to entities.