Improve GTN link previews by showing content / custom image previews

[hexylena]

This replaces our existing link preview data which was pretty boring (just the topic 'about') and the GTN image always being used, for more customised previews which include more data:

x	Before	After
title	Galaxy Training: RAD-Seq de-novo data analysis	Galaxy Training: RAD-Seq de-novo data analysis
description	Learn to analyse Ecological data through Galaxy	Introduction In the study of Hohenlohe et al. 2010, a genome scan of nucleotide diversity and differentiation in natural populations of threespine stickleback Gasterosteus aculeatus was conducted.
image	/training-material/assets/images/GTNLogo1000.png	https://training.galaxyproject.org/training-material/topics/ecology/images/RAD4_Population_Genomics/Hohenlohe_et_al_2010.png

open to further suggestions or improvements! If we merge this, we should finally remove all of the useless Introduction sections, we know it's always the bit above the Agenda, it doesn't really need to be labelled.

Here's the old preview card in Telegram, but we'll still have the "Galaxy Training Network" in big bold text at the top, now we'll just show: a better description, and a potentially more interesting image.

I can imagine we can use this "text preview" and "first image" as useful filters elsewhere in the GTN, maybe with hovercards or just providing tutorial blurbs in a future tutorial list interface.

[hexylena]

Per discussion with @shiltemann, some of the images chosen weren't always great (admin/ansible-galaxy was an SSL warning image :upside_down_face:), and we'd probably need to override at the topic+tutorial level, like we do with the news posts. So dropping the new image for now, just using the GTN picture.

[hexylena]

I think I was the only one that cared about this, so, cancelling it.