Adding plugin code from edx-platform

[jinder1s]

PR in edx-platform that tests this move: https://github.com/edx/edx-platform/pull/24678

previous location in edx-platform: https://github.com/edx/edx-platform/tree/master/openedx/core/djangoapps/plugins

This is mostly a drop in replacement, though with some link changes and I ran black on plugin directory.
Link to working sandbox: https://plugless.sandbox.edx.org/

Description:

Describe in a couple of sentence what this PR adds

JIRA:

XXX-XXXX

Dependencies:

List dependencies on other outstanding PRs, issues, etc.

Merge deadline:

List merge deadline (if any)

Installation instructions:

List any non-trivial installation
instructions.

Testing instructions:

1. Open page A
2. Do thing B
3. Expect C to happen
4. If D happened instead - check failed.

Reviewers:

• @edx/arch-review (Required)
• tag reviewer

Merge checklist:

• All reviewers approved
• CI build is green
• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Commits are squashed
• PR author is listed in AUTHORS

Post merge:

• Create a tag
• Check new version is pushed to PyPi after tag-triggered build is
  finished.
• Delete working branch (if not needed anymore)

Author concerns:

List any concerns about this PR - inelegant
solutions, hacks, quick-and-dirty implementations, concerns about
migrations, etc.

[robrap]

Thanks @jinder1s.

1. I didn't really look at code, because I presume you didn't touch it. Let me know if you made changes and what I should look for.
2. I don't have any background on "Plugin Contexts" and how baked it is, and whether it should be marked Accepted in the ADR and should be moved along with everything else. :)

UPDATE: Looks like "Plugin Contexts" are marked as Adopt, so we should be all set in moving it as well. See https://github.com/edx/edx-platform/pull/24057/files#r465341119

[robrap]

Some final details before approval:

• See comment
• Temporarily omit files from codecov with TODO comment pointing to ticket that will add coverage.
• Codecov has been having issues on this repo. Is that a global issue, or does config for this repo need to be reviewed, or do we need to open a ticket with them?
• Please add PR comment when you have successful sandbox using plugin.

[jinder1s]

• Temporarily omit files from codecov with TODO comment pointing to ticket that will add coverage.

Can you elaborate more on what you mean by "add coverage"? @robrap
Also, by omit files, you mean add plugins directory to omit in .coveragerc. Or should I comment out the patch section in codecov.yml?

[robrap]

@jinder1s: I mean adding tests.

[robrap]

You could add the pragma for no cover (search for ‘pragma’) to the if statement below to bump up coverage for the project.

38 | 4 | declared_middleware = getattr(settings, 'MIDDLEWARE', None)
39 |   | if declared_middleware is None and django.VERSION[0] < 2:
40 |   | declared_middleware = settings.MIDDLEWARE_CLASSES # Pre-Django 2 support

[robrap]

Once you get codecov to green and have tested in LMS.