Chore: Add GitHub issue templates

[machikoyasuda]

closes #604

Discussed at today's Developer Team meeting.

What this PR does

Adds 5 (!!) more issue templates for the following scenarios:

1. bug report: for anyone to report bugs on the app (to be closed with a corresponding pull request)
2. user story: for product to write user stories for new features
3. task: task is for an individual engineering task (to be closed with a corresponding pull request)
4. deliverable: "deliverable" is our new word for any github issue that's not for a code change. it could be an issue to track work like: user testing, user research, writing planning documents for a meeting. these issues don't end up with a pull request.
5. epic decision record: this is for our big "epic" issues, which track a group of related user stories, tasks and deliverables. epics can also contain relevant background information, checklist of items that need to be done before the feature can be delivered. epics should have a defined "definition of done", so we know when we can close it.

[machikoyasuda]

@thekaveman @angela-tran I drafted these up! Please comment on any things I should remove, add, change (Or send up your own commits making these changes).

[thekaveman]

Thanks for getting this started @machikoyasuda! These look great. Overall I think we can merge sooner than later and refine these as we go and learn more.

I did make a few minor cleanups, reorganizations, heading levels, etc. I also created the deliverable label.

[angela-tran]

These look great!