Simplify CI infrastructure

The ROCm and Intel images will no longer be tested in CI in 4.2.

ROCm 3.7

Create CUDA 11 image

Use the NVIDIA-provided base image.

Remove doubly-defined python3-lxml and add --no-install-recommends.

Install coverage python package in for ubuntu.

Update images

Update images to latest stable versions

Add Stokesian Dynamics dependencies in ROCm image

This guarantees apt fetches all ROCm packages from the same release.

Install llvm-symbolizer

Debug builds with ASAN require llvm-symbolizer to generate reports.

Reduce number of CUDA images

GitHub runners are limited to 7 GB of RAM, which limits docker image
sizes to at most 6.6 GB. Removing suggested packages saves 1.8 GB in
the Ubuntu 20.04 image.

Make the Ubuntu 20.04 image CUDA-capable

Add ffmpeg for tutorial 11 ferrofluid

Added badge to README.md.

Sphinx downgrade and Travis removal

ubuntu-travis and ubuntu-18.04 are nearly identical. They are both
accessible to Travis-CI from DockerHub. Using image Ubuntu 18.04
in both GitLab-CI and Travis-CI reduces dockerfile duplication.

There is a bug that will be fixed in 3.0.1.