Issue #82: Refactors and cleanups

Removed STRIPE_PAYMENT_PRODUCT_NAME as unused, removed stripe_payments.py constants, removed redundant js scripts.
Added extra project tools for scripts and styles: linters, prettifier (in progress).
Updated project configuration, calmed linters warnings, updated sources to meet prettifier rules.
Removed urls definitions from dds_registration/urls/__init__. Updated names of urls modules (to make them more distinctive from the same in views package).
Removed and calmed debugger linter warnings.

sentier-dev / dds_registration