Auto merge of #9970 - ehuss:issue-template-forms, r=alexcrichton

Use forms for issue templates.

This switches the GitHub issue templates to use the new forms functionality.  This makes it so that the user doesn't need to fuss with editing the placeholder text, and instead can just start writing text.

I'm mostly just curious how this will work out. If people have trouble with it, or there are complaints, it can be reverted pretty easily.