issues: add homeserver to template

.gitlab/issue_templates/Bug.md

@@ -14,6 +14,8 @@ Steps to reproduce:
 
 **Installation Source**: <!-- Where you installed Fractal from, e.g. Flathub, AUR, or distro repositories -->
 
+**Homeserver**: <!-- The homeserver for your matrix account, e.g. matrix.org, gnome.org, … You can mention several of them if this is reproducible on multiple ones. -->
+
 <!-- If you have a crash report, use the "Attach A File" button in the issue editor to attach it, or paste it in a code block below. -->
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