Allow changing the default template for revert PRs

[seanbudd]

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When performing a PR revert, GitHub automatically generates a PR description.
This overrides the default template for the project, removing vital information expected in a pull request.
The current template is as follows:

Title: "Revert "$PRTITLE""
Body: "Reverts $PRNUM"

As developers, it is often worthwhile to include more information in the revert.
For example, in nvaccess/nvda we have created a template body as follows:

### Reverts PR
Reverts #issueNumber

### Issues fixed
<!-- Issues that will be closed by reverting, i.e. the issues introduced via the PR getting reverted  -->
Fixes #issueNumber

### Issues reopened
<!-- Issues that will be re-opened by reverting, i.e. the issues that were fixed via the PR getting reverted  -->
Reopens #issueNumber

### Reason for revert

### Can this PR be reimplemented? If so, what is required for the next attempt

This needs to be copy pasted into the PR body.
It would be helpful to be able to override GitHubs default revert template, as it overrides the projects default PR template.