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.

• 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  • "Snooty language server is not installed or out of date."