Refers EP-126 -- Add Celery setup for delayed tasks
Refers EP-126 -- Add bare celery tasks for invoice generation
Refers EP-126 -- Add Celery tasks for invoice export and send.

As of now the export is still in JSON format but uses the .pdf file
extension already. This is just for simplicity.
Merge branch 'develop' into feature/invoice
Merge branch 'develop' into feature/invoice
Integrated pyinvoice
Integrate basepdf for invoice rendering
Provide optimize_media_images command

This command offers a simple way to process jpegs and pngs stored
within the media root folder with optipng and jpegoptim.

Optionally, this can be limited to thumbnails and the state can
be stored to prevent double processing of the relevant files.
Make optipng/jpegoptim paths configurable.
Pass font configuration and modification password to pyinvoice
Merge branch 'develop' into feature/invoice

Conflicts:
	pyconde/settings.py
Use app settings for invoice config
Use PROJECT_NAME instead of hardcoded path
Fix escaping in thumbnail regexp.
EP-477: Add custom resize option to sponsor logo.
Fix some (missing) translations.
Refers EP-126 -- Add admin actions for invoice export/send and tempalte for invoice mail
Refers EP126 -- Move purchase receiver email fully formatted to Purchase model
Refers EP-126 -- Add "Resend Invoice" button to purchase list
Change ep14.org to europython.eu