config: Added "jinja" as additional template renderer option
setup.py: Added *.jinja files for i18n message extraction
config: Fixed {% trans %} tag usage for tg2

The Jinja2 environment is not properly configured for i18n by the tg2
framework, so we have to do that ourselves. We do that in the
"after_config" hook, after the Environment has been created and is part
of the app_globals.
templates: Added Jinja2 base templates
templates: Converted "About" page to Jinja2
templates: Converted flight details templates to Jinja2