Do not add links to Posters or Assignees with ID < 0 (#20577)

There are several places in templates/repo/issue/view_content/comments.tmpl where links are made to Posters or Assignees who are Ghosts or have IDs <0.

Fix #20559

Signed-off-by: Andrew Thornton <art27@cantab.net>

templates/shared/user/authorlink.tmpl

@@ -0,0 +1,3 @@
+<a class="author"{{if gt .ID 0}} href="{{.HomeLink}}"{{end}}>
+	{{.GetDisplayName}}
+</a>