Simplify enhancement issue template

[choldgraf]

This further simplifies our enhancement issue template, with the goal of making it easier for people to follow the guidelines by making the template structure less noisy and opinionated. Here are the major changes:

• Pre-existing field values have been removed, so people can insert whatever structure they like.
• The three sections should be a bit more self-contained now. They are Context, Proposal, and Tasks/Updates.
• Simplified instructions for each, to reduce the amount of wording on the page.

[choldgraf]

Merging this in as there didn't seem to be any objections after a couple weeks