I notice that people largely ignore PR templates, *especially* big ones.
I think that shrinking this down to just the essentials will increase the
likelihood that people use it.

In particular, I want to remove things like "All reviewers approved" and
"CI build is green" that are generally already enforced by repo policies,
"Delete working branch" (there's a setting for that if we want it!), and
release instructions (should be in a central document linked from the
README).

Co-authored-by: Kyle McCormick <[email protected]>

- Emphasize "or" in prelude
- Remove conventional commits link
- Add unit tests, manual testing; remove "testing instructions" from
  catchall
- Prune irrelevant items by type: No version bump or changelog for IDA; no
  manual testing for library.

app and xblock are identical, but probably should differ in some way. The
repo's own template is adjusted to match app/xblock, but without version
bump or migration note.

# Conflicts:
#	CHANGELOG.rst

…link