Update PR template to front-load issue references (napari#5659)

# Description

Currently, when PRs have long and detailed descriptions (a good
thing!!!), it's
hard to find the reference to the issue(s) that they close. This updates
the
template so that those issues are linked right at the top, before the
description.

I think a lot of the stuff at the bottom of the PR template is cruft (it
was
created a long time ago), but we can discuss that in separate
issues/PRs.

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,20 +1,25 @@
+# Fixes/Closes
+
+<!-- In general, PRs should fix an existing issue on the repo. -->
+<!-- Please link to that issue here as "Closes #(issue-number)". -->
+Closes #
+
 # Description
 <!-- What does this pull request (PR) do? Why is it necessary? -->
 <!-- Tell us about your new feature, improvement, or fix! -->
 <!-- If your change includes user interface changes, please add an image, or an animation "An image is worth a thousand words!" -->
 <!-- You can use https://www.cockos.com/licecap/ or similar to create animations -->
 
+# References
+<!-- What resources, documentation, and guides were used in the creation of this PR? -->
+
 ## Type of change
 <!-- Please delete options that are not relevant. -->
 - [ ] Bug-fix (non-breaking change which fixes an issue)
 - [ ] New feature (non-breaking change which adds functionality)
 - [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
 - [ ] This change requires a documentation update
 
-# References
-<!-- What resources, documentation, and guides were used in the creation of this PR? -->
-<!-- If this is a bug-fix or otherwise resolves an issue, reference it here with "closes #(issue)" -->
-
 # How has this been tested?
 <!-- Please describe the tests that you ran to verify your changes. -->
 - [ ] example: the test suite for my feature covers cases x, y, and z