add support for relative links to images

[neuromusic]

Who it's for

Users and plugin developers

Problem it solves

Whether a plugin developer uses .napari-hub/DESCRIPTION.md or their PyPI description (which often copies README.md), Github supports relative links for images, but then they are not displayed on the napari hub.

How it will work

When the plugin developer has provided relative links, we will attempt to resolve them if they have provided links to a valid Github repo.

[codemonkey800]

PR for this issue has been created and is on hold pending review:
#796

[richaagarwal]

@codemonkey800 Out of curiosity, is this something mostly pending code review or were their complications you identified that caused us to put it in the backlog?