gitpod usability improvements

[mkoeppe]

• in the terminal, viewing graphics silently does nothing

  sage: polytopes.cube().show()
  Launched html viewer for Graphics3d Object
  sage: polytopes.regular_polygon(5).show()
  Launched png viewer for Graphics object consisting of 7 graphics primitives
  

• The scipy folks have some good documentation and configuration (​​https://scipy.github.io/devdocs/dev/contributor/quickstart_gitpod.html), perhaps we can lift something from there.

• The docker image is quite big and thus it takes long to open a new gitpod workspace. (from ​https://trac.sagemath.org/ticket/33103#comment:19): could use docker/.gitpod.Dockerfile again with COPY --from to make the final image smaller (e.g., currently /home/gitpod/sage has the old source tree) Also installation of system packages should make use of the install-packages script ​https://github.com/gitpod-io/workspace-images/blob/master/base/install-packages which does a bit of cleanup after the installation.

• Make docker image smaller by using mv instead of cp when installing packages via DESTDIR staging (​https://trac.sagemath.org/ticket/29585#comment:5)

• Add gitpod badge to trac tickets: Image https://img.shields.io/badge/-Gitpod-908a85?logo=gitpod Link https://gitpod.io/#https://github.com/sagemath/sagetrac-mirror/tree/<branch>

• Remove popup notifications:
  • "Snooty language server is not installed or out of date."