add jobs for testing strategy branches

[mathemancer]

Fixes #3261

This PR modifies the CI pipeline testing logic in the following way:

All testing, linting, auditing, etc. jobs are now in the same workflow, called "Test and Lint Code" .

Each job is dependent on a job that checks whether relevant files have changed. For example, the front end testing job is dependent on a job that checks whether any file under mathesar_ui has changed. If such a file has changed, the job runs the tests, otherwise the job ends in a skipped state. This state is sufficient to allow PRs to merge. Thus, we no longer need dummy jobs to run so that PRs which don't change, e.g., mathesar_ui/** can be merged.

Also, all such jobs are moved into the same workflow and same workflow file. The reason for this is that:

• It makes the branching easier, and avoids repetition.
• It makes it easier to compare the file specifications under which different jobs run.

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the develop branch of the repository
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no
  visible errors.

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[mathemancer]

I've marked this as 'urgent', since the fix is blocking merging backend PRs, and the current state of branch protections necessary for properly reviewing this PR blocks merging all other PRs.

[kgodey]

Approved, review conducted on a call.