WebStyle: customize Flask request class

* Adds InvenioRequest as the default Flask request class with methods
  to allow for easier Flaskification of legacy modules.
WebSession: fix typo in variable name
WebSession: port user registration to Flask

* Ported user registration view to Flask but moving register view from
  websession_webinterface to webaccount_blueprint and creating a new
  WTForm form with validation logic for user registration.

* Use email instead of nickname for login

* Fix issue in webuser when dealing with Flask generated legacy
  requests instead of proper legacy requests.

* FIXME Unauthorized access page not ported
WebDeposit: fix pre_validate method in fields

* Ensure pre_validate function signature matches that of WTForms.
  Already fixed by Kostas, so to be removed from next-op once new
  WebDeposit changes have been pushed and integrated into next-op.
WebStyle: install I18N Jinja2 extensions

* Load Jinja2 I18N extension.
WebStyle: fix Flask request compatibility issue.