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/views/settings/tabs/user/_PreferencesUserSettingsTab.scss

@@ -15,5 +15,5 @@ limitations under the License.
 */
 
 .mx_PreferencesUserSettingsTab .mx_Field {
-    margin-right: 100px; // Align with the rest of the controls
+    @mixin mx_Settings_fullWidthField;
 }