Add Acceptance testing issue template

What does this MR do?

Adds an issue template, for standardizing the acceptance testing. Mostly lifted from Gitaly.

Does this MR meet the acceptance criteria?

• Documentation created/updated
• [-] Tests added for this feature/bug
• [-] Conforms to the code review guidelines
• [-] Conforms to the merge request performance guidelines
• [-] Conforms to the style guides
• [-] Conforms to the database guides