Differentiate icons between GitLab instances

Currently all GitLab instances share the same icon.
It can be a pretty normal thing for some users to use multiple GitLab instances, which are annoying to differentiate in Gittyup.
I would suggest that we look at the GitLab instance URLs and choose a more appropriate icon if available.
For example, I am on GitLab, on the KDE Gitlab instance, on the Gnome GitLab instance, on the Debian GitLab instance, and on the Alpine Linux GitLab instance.
We could either grab their favicons, or add logic for Gittyup for looking if icons are available, and then we could manually add them.
All of the instances that I checked actually have slightly or even very oversized favicons, so they would be an option even on high-dpi displays.

No files or tests are named. Start by tracing how Gittyup represents GitLab instances and selects their current shared icon, then inspect how instance URLs could map to available favicons. Done means distinct GitLab instance icons are shown where available without losing a fallback for instances that provide none.