Unintentional Pull Requests by people completing the task

[rockett-m]

The Pull requests tab is full of users trying to get their solutions into the main branch here. They don't seem to get that there is already a model-answer branch and PRs here are to solve issues rather than merge their solution to the task.

I assume a lead maintainer could adjust the settings for this.

1. If running git remote -v did not give an upstream by default then users would push to their local repo without the "Compare and pull request" suggestion appearing and leading to all the unintentional PRs here.
2. You could also only allow PRs if there is an issue linked.
3. Or a template for PRs like:

## Important: Do Not Submit Task Solutions
If you are trying to submit a solution to the task, **DO NOT OPEN A PULL REQUEST**. Instead, commit and push to your own fork.

This repository does not accept pull requests for task solutions. PRs should only be opened to fix issues or improve the codebase.```