Update the PR template to fix #117.

[StephanTLavavej]

Description

This adds a checkbox to the PR template mentioning
license discipline.

It clarifies the instructions regarding unchecked
and inapplicable boxes.

It fuses the "acceptance will be delayed" disclaimer
into the README checkbox.

This also rewraps the issue template (we usually wrap to 120,
but 80 is friendlier to the issue textbox) and improves the
wording by changing "Alternately" to "Alternatively".

Checklist

If you're unsure about a box, leave it unchecked. A maintainer will help you.

If a box isn't applicable, add an explanation in bold.
For example: (N/A: this is a bugfix, not a feature)

• I understand README.md. I also understand that acceptance of
  community PRs will be delayed until the test and CI systems are online.
• If this is a feature addition, that feature has been voted into the
  C++ Working Draft. (N/A: docs only)
• Identifiers in product code changes are properly _Ugly as per
  https://eel.is/c++draft/lex.name#3.1 . (N/A: docs only)
• Identifiers in test code changes are not _Ugly. (N/A: no tests)
• Test code includes the correct headers as per the Standard, not just
  what happens to compile. (N/A: no tests)
• The STL builds successfully and all tests have passed (must be manually
  verified by an STL maintainer before CI is online, leave this unchecked for
  initial submission). (N/A: docs only)
• These changes introduce no known ABI breaks (adding members, renaming
  members, adding virtual functions, changing whether a type is an aggregate
  or trivially copyable, etc.). (N/A: docs only)
• These changes were written from scratch using only this repository and
  the C++ Working Draft as a reference (and any other cited standards).
  If they were derived from a project that's already listed in NOTICE.txt,
  that's fine, but please mention it. If they were derived from any other open
  source project (including Boost and libc++, which are not yet listed in
  NOTICE.txt), you must mention it here, so we can determine whether the
  license is compatible and what else needs to be done.