Fix invite input box size in non-Firefox browsers

font-size-adjust is still a flag-enabled feature in Chrome 77, and
doesn't have widespread support across browsers. So instead this uses
font-size to make the text large enough.

Ref T690

templates/user/invite-instructions.tmpl

@@ -3,9 +3,9 @@
 <style>
   .copy-link {
     width: 100%;
-    margin: 1em 0;
+    margin: 1rem 0;
     text-align: center;
-    font-size-adjust: .7;
+    font-size: 1.2em;
     color: #555;
   }
 </style>