Simplify our issue templates

[choldgraf]

Description

Right now we have some nice default issue templates defined here: https://github.com/executablebooks/.github

They ask several questions about either enhancements or bugs to help boost the signal to noise of these issues. However, over time I've noticed that a lot of people only fill out one or two sections, and often lump content that is technically supposed to be split across sections.

I think we should consider simplifying these issue templates slightly, with the goal of:

• Having fewer unique fields to fill in
• Providing guidance and prompts for information, but only requiring the really important information
• Providing some basic structure that most issues should follow to help guide people in providing the right information

Hopefully doing this will boost the SNR of our issues and further refine our ability to quickly triage and understand them.

Thoughts?

I'll try to whip together a quick PR to propose some changes, but wanted to open this up for general discussion if people have ideas or thoughts.