Linux implementation using ffmpeg

It's difficult to support both old and new versions, so raise the
requirement to the oldest that is commonly used, which is CMake 3.10.2
on Ubuntu 18.04.

This version of Ubuntu is no longer fully supported so let's stop caring
about builds there.

Travis has unfortunately been severely rate limited by Docker Hub so we
have to switch to GitHub, which has an agreement with Docker to avoid
the rate limiting.

A initial workflow needs to be defined for actions to trigger on pull
requests, so just add a minimal, dummy one.

So that people can easily find them and not file bug reports for
things that are just questions.

We want users to use the templates so we don't miss any relevant
information.

Add basic templates for users to follow so that we get the information
we need directly in the initial report.