[chore] make single pull request template (#1239)

When you have multiple templates, they can only be selected by the API, so you can't just open a pull request via the web UI and have the template presented to you. This should fix that by having just one pull request template.

.github/pull_request_template.md

@@ -0,0 +1,26 @@
+# Description
+
+> If this is a code change, please include a summary of what you've coded, and link to the issue(s) it closes/implements.
+>
+> If this is a documentation change, please briefly describe what you've changed and why.
+
+This pull request implements xyz or fixes abc.
+
+closes #(issue)
+closes #(another issue)
+
+## Checklist
+
+Please put an x inside each checkbox to indicate that you've read and followed it: `[ ]` -> `[x]`
+
+If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).
+
+- [ ] I/we have read the [GoToSocial contribution guidelines](https://github.com/superseriousbusiness/gotosocial/blob/main/CONTRIBUTING.md).
+- [ ] I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
+- [ ] I/we have performed a self-review of added code.
+- [ ] I/we have written code that is legible and maintainable by others.
+- [ ] I/we have commented the added code, particularly in hard-to-understand areas.
+- [ ] I/we have made any necessary changes to documentation.
+- [ ] I/we have added tests that cover new code.
+- [ ] I/we have run tests and they pass locally with the changes.
+- [ ] I/we have run `go fmt ./...` and `golangci-lint run`.