Refactored tasks files from project & core

[shubham76]

This is regarding #3775.
Moved the mentioned tasks from projects/tasks.py to core/tasks.py.
Also made some changes in docs, tests & a few other files where the call was being made to projects/tasks.py.
Please have a look and let me know if something is wrong.

[agjohnson]

Thanks for taking a swing at this! I think we may have missed an opportunity to talk about this first, so we likely sent you down the wrong path. Hold up on changes here for now, I've raised issues in the underlying ticket for this PR.

A few other things:

• Moving code and altering code makes this near impossible to review, so it might be best to split up the work to refactor that location with a separate PR with the code changes you applied. Alternatively, you can move code in one commit and change code in another commit, and mention that on the PR.
• Tests appear to be broken, but I'd wait on more guidance on the underlying ticket before continuing to resolve the errors here, as reverting this will require more code changes

[shubham76]

Deleted the branch as there was a lot of mess in the files. Created another branch to make changes as suggested.