Reapply "Allow for both hexcolors with alpha as shorthand"

This reverts commit 02e5dd5.

Author: vmcj

webapp/src/Utils/Utils.php

@@ -290,7 +290,10 @@ public static function timeStringDiff(string $time1, string $time2): string
      */
     public static function convertToHex(string $color): ?string
     {
-        if (preg_match('/^#[[:xdigit:]]{3,6}$/', $color)) {
+        if (preg_match('/^#[[:xdigit:]]{3,4}$/', $color) ||
+            preg_match('/^#[[:xdigit:]]{6}$/',   $color) ||
+            preg_match('/^#[[:xdigit:]]{8}$/',   $color)
+        ) {
             return $color;
         }