Allow images from GitHub domain on the wiki

[emyoulation]

@gramps-project

Checked the wiki and see that I already added urls for github and gramps discourse back in May 2021 , but as @Gioto points out the only format accepted using this method are bare (plain) urls (but you can embed the url in a table).

I tried previewing bare URL tests for a .png from Kaj’s HistContext repository. And the wiki just rendered the URL…maybe ‘Preview’ isnt enough?

https://github.com/kajmikkelsen/HistContext/blob/master/gramplet.png

(Discourse tries but says the bare URL is a binary file. So had to wrap the URL with a link markup.)

kajmikkelsen/HistContext/blob/master/gramplet.png