Improve PR and Issue templates.

[n2ygk]

Description of the Change

• Add instructions as template comments.
• Fix misunderstanding of how labels work.

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• CHANGELOG.md updated (only for user relevant changes)
• author name in AUTHORS

[coveralls]

Pull Request Test Coverage Report for Build 1353

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 94.795%

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Totals
Change from base Build 1349:	0.0%
Covered Lines:	1293
Relevant Lines:	1364

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💛 - Coveralls

[n2ygk]

Ugh, travis status is broken. It finished no problem: https://travis-ci.org/github/jazzband/django-oauth-toolkit/builds/666809770

[n2ygk]

Ugh, travis status is broken. It finished no problem: https://travis-ci.org/github/jazzband/django-oauth-toolkit/builds/666809770

From https://www.traviscistatus.com/:

GitHub status may not be posted on commits occasionally from builds using the legacy Services integration

Investigating - We are investigating the reports of GitHub commit status not being posted.

The issue only affects those repositories using the legacy Services integration (i.e., all .org repositories and some .com ones), and only under some circumstances. You can migrate your repositories to GitHub Apps on https://travis-ci.com/account/repositories.