Update pull request template (#13908)

In order to properly review a pull request, it is necessary to not only
understand the details about the changes presented, but also the
*context* behind those changes. Oftentimes, this context is difficult to
locate or even absent altogether. Providing a link to an issue or a user
story helps, but only if there are sufficient details listed in the
resource itself, which may not always be the case. Ultimately, I feel
that the best way to provide such information is to briefly explain it
in the PR description.

Additionally, for changes that involve UI/UX improvements, screenshots
or screencaps can really help the review process, as one can comprehend
the changes much faster than reading the relevant code.

With that in mind, this commit updates the pull request template to
include some more prompts and a better template for the description
area.

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,8 +1,47 @@
-Fixes: #
+## Explanation
 
-Explanation:  
+<!--
+Thanks for the pull request. Take a moment to answer these questions so that reviewers have the information they need to properly understand your changes:
 
-Manual testing steps:  
-  - 
-  - 
-  - 
+* What is the current state of things and why does it need to change?
+* What is the solution your changes offer and how does it work?
+
+Below is a template to give you some ideas. Feel free to use your own words!
+
+Currently, ...
+
+This is a problem because ...
+
+In order to solve this problem, this pull request ...
+-->
+
+## More information
+
+<!--
+Are there any issues, Slack conversations, Zendesk issues, user stories, etc. reviewers should consult to understand this pull request better? For instance:
+
+* Fixes #12345
+* See: #67890
+-->
+
+## Screenshots/Screencaps
+
+<!-- If you're making a change to the UI, make sure to capture a screenshot or a short video showing off your work! -->
+
+### Before
+
+<!-- How did the UI you changed look before your changes? Drag your file(s) below this line: -->
+
+### After
+
+<!-- How does it look now? Drag your file(s) below this line: -->
+
+## Manual testing steps
+
+<!--
+How should reviewers and QA manually test your changes? For instance:
+
+- Go to this screen
+- Do this
+- Then do this
+-->