Format the index as columns when possible

To avoid wasting space.

irctest/dashboard/format.py

@@ -277,6 +277,7 @@ def write_html_index(output_dir: Path, pages: List[Tuple[str, str]]) -> None:
     ET.SubElement(body, "h1").text = "irctest dashboard"
 
     dl = ET.SubElement(body, "dl")
+    dl.set("class", "module-index")
 
     for (module_name, file_name) in sorted(pages):
         module = importlib.import_module(module_name)

irctest/dashboard/style.css

@@ -8,6 +8,10 @@
     }
 }
 
+dl.module-index {
+    column-width: 40em; /* Magic constant for 2 columns on average laptop/desktop */
+}
+
 /* Only 1px solid border between cells */
 table.test-matrix {
     border-spacing: 0;