Change `license.file` to `license-files` + SPDX-compliance (#528)

I encountered this issue when making a release
astro-informatics/sleplet#451. Also, make
`BSD-3` SPDX-compliant.

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Co-authored-by: Matt Graham <matthew.m.graham@gmail.com>

Change `license.file` to `license-files` + SPDX-compliance (#528)

I encountered this issue when making a release
astro-informatics/sleplet#451. Also, make
`BSD-3` SPDX-compliant.

---------

Co-authored-by: Matt Graham <matthew.m.graham@gmail.com>

Renaming `github_username` to `github_owner` + generating `__repo_nam…

…e` & `__repo_url` (#409)

Though its not exposed to user now we have longer prompts rather than
using variable names directly, I think `github_username` is a bit of a
misnomer given that it can be either a user or organization name, hence
I think `github_owner` would be a better choice.

We also currently manually construct the GitHub repository URL in lots
of different bits of the template and also the qualified repository name
in a few places. We can use [double underscore prefixed (rendered)
private
variables](https://cookiecutter.readthedocs.io/en/stable/advanced/private_variables.html)
in the cookiecutter config to generate the repository URL and qualified
name once and then reuse elsewhere to avoid the repetition. This would
also make it simpler to later switch to supporting alternative
repository hosting options such as GitLab in future (which is the
rational for naming the variables `__repo_name` and `__repo_url` rather
than `__github_repo_name` and `__github_repo_url`).

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Co-authored-by: Patrick J. Roddy <patrickjamesroddy@gmail.com>
Co-authored-by: David Stansby <dstansby@gmail.com>