navbar: Rename tab_bar to message_view_header.

Prior to commit eb4a2b9d4e the center
area of the navbar was based on a structure that appended crumbs or
"tabs" as <li>s, forming a tab_bar and a tab_list.

However, in eb4a2b9d4e we apply a new
style and structure to the navbar which lets go of the convention of
tabs. Hence, we'd like to purge the tab_bar and tab_list labels from
our code base.

We purged tab_list in 1267caf5009118875f47fdafe312880af08024e1.

This commit purges tab_bar, it includes:
- A blanket search and replace of tab_bar with message_view_header.
- Splitting a single line comment in
  tab_bar.js / message_view_header.js.
- The renaming of tab_bar.js to message_view_header.js.
- The renaming of tab_bar.hbs to message_view_header.hbs.
- A blanket search and replace of tab_data with
  message_view_header_data.
- Replacing the single occurrence of tabbar with message_view_header
  (it was within a comment.)

static/js/floating_recipient_bar.js

@@ -3,7 +3,7 @@ const XDate = require("xdate");
 let is_floating_recipient_bar_showing = false;
 
 function top_offset(elem) {
-    return elem.offset().top - $("#tab_bar").safeOuterHeight();
+    return elem.offset().top - $("#message_view_header").safeOuterHeight();
 }
 
 exports.first_visible_message = function (bar) {