Leverage Jinja better

[jaimergp]

We were depending already on Jinja to parse the input file, so using it with our templates was a no brainer.

Some items:

• Use Jinja for templating #892. This is the initial 1:1 conversion, not trying to be too smart to keep the diff digestable.
• Move some simple templating logic like .lower() from the Python modules over to the Jinja template, where they belong
• Replace some variables that render lots of lines with the corresponding for-loops in JInja

[michalwyszynski93]

examples/custom_nsis_template/custom.nsi.tmpl should also be changed to jinja