Fix crash on PRs made by deleted users and add icon for discussion notifications

This PR includes a couple of small changes that IMO would be worthwhile to include before the next release: - fix a crash that occurs on PRs made by deleted users due to the `head` repository object having a `null` owner. There are several closed PRs in [azahar-emu/azahar](https://github.com/azahar-emu/azahar) repo which trigger this crash in the current version of the app. - add an icon for discussion notifications in the notifications list (previously the app didn't show any icon for those). I've chosen to use the Material ["forum" outline icon](https://fonts.google.com/icons?selected=Material+Icons+Outlined:forum:&icon.set=Material+Icons&icon.size=24&icon.color=%231f1f1f) which is the one that most resembles [the icon used by GitHub](https://primer.style/octicons/icon/comment-discussion-16/). <img src="https://github.com/user-attachments/assets/fa514237-998e-4c7f-9754-9e2db5bbed25" width=415 />