[Proposal] Display related PRs using icons rather than text

[smallsaucepan]

Currently we display related PRs as text:

It might be beneficial to represent these using the same icons used on github, which feels more polished and conveys more information at a glance:

Discussion to make sure it can be done accessibly: https://forum.djangoproject.com/t/ensuring-trac-related-prs-idea-is-accessible/45761