flask: Replace create_app() function by SkyLines class
flask: Added inject_tg2_compat() method
flask: Added register_views() method
flask: Added comments
flask: Added wrap_toscawidgets_middleware() method
flask: Simplified WSGI server run call
flask: Added add_*() methods
flask: Moved extension imports into methods

There is no need to import these modules if we don't use them
flask: Added add_web_components() method
flask: Call register_views() from add_web_components()
flask: Converted comments to docstrings
flask: Moved add_web_components() call out of skylines package

We can now use the app instance with only SQLAlchemy extension setup
and without the views and other cruft usually attached to it
flask: Export db variable
views: Added /files route
model/igcfile: Removed unnecessary get_download_uri() method

.. and with it the last flask dependency in the model
templates: Removed usage of tmpl_context and c
config: Added to_envvar() function

This can be used in combination with the ArgumentParser of many command
line scripts to specify the config file.
tracking/server: Use DBSession instead of transaction

We don't have/need multi-database support at the moment...
config: Removed outdated tg2 files
middleware: Removed outdated tg2 files