admin: UI elements implementation
admin: UI elements implementation

* NEW Adds new header, main sidebar-menu and footer to the admin page.

* Adds temporary solution for adding icons to category items
  displayed in the main sidebar-menu. Currently not supported
  in Flask-Admin. (addresses flask-admin/flask-admin#898)

* Improves kwalitee.

Signed-off-by: Jochen Klein <758ae21bb63ff91b34fd67a62ea6f5f6075466ad@cern.ch>

inveniosoftware/invenio-search,inveniosoftware/invenio-search,tiborsimko/invenio-search,tiborsimko/invenio-search