base: new signal `before_handle_user_exception`

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
pages: 404 exception handling

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
base: fix admin template

Signed-off-by: Javier Martin Montull <javier.martin.montull@cern.ch>
pages: model improvements

* Adds support for urls without trailing slash.

* Adds validation for template name.

* Adds last_modified and created columns.

* Improves upgrade recipe.

Signed-off-by: Javier Martin Montull <javier.martin.montull@cern.ch>
Co-authored-by: Jiri Kuncar <jiri.kuncar@cern.ch>