WebStyle: 1st commit of Flask-Invenio bridge

• New Flask-Invenio bridge to serve web request via Flask.

• Ports Invenio page layout to Jinja2

• Implements Flask blueprint support via pluginutils. Every handler will be implemented as a blueprint using dedicated Invenio decorators.

• Support for I18N, navtrails.

• Invenio session is shared between Invenio web handler and Flask handler.