settings: Add "text" option to emoji_set model.

We no longer have a special UI setting and model
field ("emoji_alt_code") for saying users want text-only
emojis.  We now instead make "text" be a fifth choice
for "emojiset".

Fixes #7406

static/js/compose.js

@@ -820,7 +820,7 @@ exports.initialize = function () {
             preview_html = rendered_content;
         }
         $("#preview_content").html(preview_html);
-        if (page_params.emoji_alt_code) {
+        if (page_params.emojiset === "text") {
             $("#preview_content").find(".emoji").replaceWith(function () {
                 var text = $(this).attr("title");
                 return ":" + text + ":";