Drop Travis and AppVeyor Support

[Lnaden]

Due to a recent change to Travis Billing (specifically dropping macOS support)[https://blog.travis-ci.com/2020-11-02-travis-ci-new-billing], we've had to update our timetable for when to drop support for Travis and AppVeyor.

This PR should scrub the two CI systems from options, documentation, output, etc. while also explaining why we opted to do this.

There isn't a migration guide yet, but I think that will take time to properly get in place I cannot commit to this week.

I renamed and kept the old "before install" file from the Travis CI setup because it was such a useful script I didn't want to wholesale do away with it (although its rendered moot by the GitHub Action). If someone else wants to get rid of it, they can.

Partial resolution to #122, still need a migration guide (which can be in a later PR if time is of the essence here)

[lgtm-com[bot]]

This pull request introduces 1 alert when merging 2eaed733f740864a76a55193d4b66a2bff3029a9 into 73d506dc5ec729c10ffdbc96898307131a816a07 - view on LGTM.com

new alerts:

• 1 for Unused import

[Lnaden]

And this PR will not pass its AppVeyor or Travis tests, something I will be disabling once its merged in.

[janash]

Making a list of changes I noted in this PR, to better see if anything has been forgotten:

• GHA reference workflows python versions updated (3.6 and 3.7 to 3.7 and 3.8)
• project names changed in GHA
• travis yml file removed
• Travis CI and Appveyor badges removes from cookiecutter README. GHA badge added.
• Other parts of readme updated
• travis-ci directory removed (under devtools)
• appveyor yaml removed
• continuous integration option removed from cookiecutter json
• updates to cookiecutter docs

Very slight thing:

• "agnostic" is mispelled in the README as "anostic" (description of create_conda_env.py file). Looks to be carried through from before.

Thoughts:

• If we are including before_install.sh in a legacy folder, should we also include the old travis yaml? You did address it in your description of the PR. Would it be useful to anyone who ran the CookieCutter fresh, and if not, why not just get rid of it? I image it will still be in previous versions.

• I ran this a few times locally, once choosing pip only for the dependencies. In this case, devtools/conda-envs/test_env.yaml is still included, even though it is not used for testing. Is there a reason for this? (unrelated to current PR).

Looks like a thorough removal or Appveyor and Travis to me. I am able to run it locally and the GHA tests are passes so it lgtm.