Make mixin for fields in settings that need to be the same width

and make that width narrower so we can fit a tooltip in the left
hand side (they were a little too wide anyway for the kind of data
being entered, even on a narrow window).

res/css/_common.scss

@@ -559,3 +559,7 @@ input[type=text]:focus, input[type=password]:focus, textarea:focus {
 .mx_Username_color8 {
     color: $username-variant8-color;
 }
+
+@define-mixin mx_Settings_fullWidthField {
+    margin-right: 200px;
+}