settings: Calculate height of create modal through javascript.

We need this height to be flexible based on if the subheader has
overflowed into two rows or not, and our method of doing this
(with existing examples in the buddy list and stream list) is to
calculate what the height should be when a window is loaded or
resized.

web/styles/app_variables.css

@@ -582,8 +582,6 @@
        in subscriptions and user group settings overlays */
     --settings-overlay-subheader-height: 3.1429em; /* 44px at 14px/em */
     --settings-overlay-header-button-height: 2em;
-    /* .settings-sticky-footer */
-    --subscriptions-overlay-sticky-footer-height: 60px;
     /* Informational overlay */
     --informational-overlay-max-width: 43.75em;
     --modal-input-width: 23.2142em;