* refactor: Private images rendering in PDFs
Private images currently render fine if PDF is generated during a
request as we pass the cookiejar to WKHTML.
Background jobs however fail completeley because they can't retrieve
private images without cookiejar.
This PR converts all image types to base64 encoded sources in HTML
itself, so wkhtmltopdf doesn't have to a fire a request.
* test: private images in pdf
The license.txt file has been replaced with LICENSE for quite a while
now. INAL but it didn't seem accurate to say "hey, checkout license.txt
although there's no such file". Apart from this, there were
inconsistencies in the headers altogether...this change brings
consistency.
* Remove six for PY2 compatability since our dependencies are not, PY2
is legacy.
* Removed usages of utils from future/past libraries since they are
deprecated. This includes 'from __future__ ...' and 'from past...'
statements.
* Removed compatibility imports for PY2, switched from six imports to
standard library imports.
* Removed utils code blocks that handle operations depending on PY2/3
versions.
* Removed 'from __future__ ...' lines from templates/code generators
* Used PY3 syntaxes in place of PY2 compatible blocks. eg: metaclass
