web interface: jinja2 with bootstrap 4

Hi all,

I'm new to this list, I hope this topic wasn't covered before. We
started to use roundup for our issue tracking and I felt the web
interface could need a bit of a "face lift".

As I'm unfamiliar with most of the technology used around roundup I
switched to what I know. If anyone is actually interested in the work
I'm happy to change some tools.

My plan is to upgrade the jinja2 rendering engine to use Bootstrap 4.x
and update the jinja2 config.ini, which seems outdated and currently
causes trouble when chosen.

I never touched mercurial before so I decided to mirror the code on
GitHub, please let me know if this troubles any of roundups licenses.

The progress is available here:
https://github.com/aparcar/roundup

If anyone is interested in these changes I'm happy to look into
mercurial and send pull requests.

Best,
Paul