Let ZIP import automatically handle common off-the-shelf theme scenarios

[fulv]

Currently, when you import a zip file for a non-Plone theme, several steps need to be done manually. That is even in the best-case scenario where a theme is actually quite well-behaved right off the shelf, e.g. startbootstrap.

Two things would be very easy and risk-free to implement:

1. If a manifest.cfg file is not present in the zip file, create it automatically.
2. Automatically parse the src and href attributes in the index.html file to fix the paths to any images, and other resources.

I was hoping to implement this at the ploneconf sprint, but ran out of time. I am still planning on it in the next few weeks.

@pigeonflight @jean @ebrehault @datakurre

[cewing]

@fulv I've added this as a possible project idea for GSoC 2017. I've listed you as the mentor for it. Please read my write up (https://plone.org/community/gsoc/ideas) and let me know if you want to make any edits.