Improve GitHub Issue Templates

[Matt Wang]

We should update our issue templates to:

• emphasize reproducibility (ideally through the template!), with a better "expected" versus "actual" split.
• make the user agent information more concise (this is the current "desktop"/"smartphone" split)
• include a note that users should search for documentation and existing issues first!
• (optional): add a checkbox for users looking to submit a PR to fix the issue

Ref: #1327 (comment).