MAINT: Simplify issue templates

[choldgraf]

This attempts to slightly simplify our issue templates. It doesn't make any huge changes, but does a few things to clean them up:

• Reduces the total number of fields for people to fill out, and instead gives a few prompts within fields that they can fill out if they find it useful. This should reduce the number of no response entries in our issues.
• Structures the information prompts based on some general best practices for issues (see this blog post for one example).

I was also considering lumping together bug and enhancement since a lot of the most relevant information is the same, but decided that it'd make things too complex to lump together all the "report your environment / reproduce the bug" stuff together in the same template. Curious what others think though.

fixes https://github.com/executablebooks/meta/issues/633

[mmcky]

@choldgraf I think these are great improvements 👍

[choldgraf]

ok cool - I'll merge this in and we can continue iterating on these templates if there is more feedback!