templates/about: Improved layout and added more information
templates/about: Added banner image
templates/about: Modified welcome text
templates/about: Use id="about" instead of "getting_started"
templates/about: Added social buttons
templates/about: Added links to tracking and database headers
templates/about: Added icons to external links
views: Let / route serve the about page
Merge pull request #87 from Turbo87/about

Improved "About" page
i18n: Updated messages.pot template file
templates/about: Adjust column layout for lead line
style.css: Made "(planned)" labels smaller

.. to fit longer headlines in translations
i18n: Updated German translation
Merge pull request #88 from Turbo87/i18n

Updated translations
Use Celery for asynchronous tasks.

Needs a running redis server - see INSTALL.md for short instructions.
celery_worker: Use config.to_envvar for command line config
worker: Added analyse_flight task
scripts: Added analyse_delayed.py
config: Renamed CELERY_BROKER_URL setting to BROKER_URL

see Celery documentation
views.upload: use async task for in-depth reanalysis of uploaded flights