navbar: Extract tab_bar anchor out of the search divs.
navbar: Extract tab_bar anchor out of the search divs.

Prior to commit eb4a2b9d4e80d495bf0a11c87bb31d18e94e9cc0 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 eb4a2b9d4e80d495bf0a11c87bb31d18e94e9cc0 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. This commit pushes us towards that goal.

Previously, this element was part of both searchbox and
searchbox_legacy of which, only one would render based on the flag on
search pills. This wasn't great because:
* it made it likely that someone would change only one of the two and
  unintentionally introduce regressions.
* it meant that search_icon selectors within the searchbox would mess
  with the search_icon elements within the tab_bar, leading us to rely
  on messy CSS overriding.

Since there doesn't seem to be a strong reason to have this be the way
it previously was, this commit extracts "#tab_bar".

It's worth keeping in mind that we use the "#tab_bar" element as an
anchor to append the #tab_list onto.

punchagan/zulip,kou/zulip,eeshangarg/zulip,rht/zulip,kou/zulip,hackerkid/zulip,showell/zulip,eeshangarg/zulip,rht/zulip,brainwane/zulip,andersk/zulip,kou/zulip,rht/zulip,brainwane/zulip,zulip/zulip,hackerkid/zulip,eeshangarg/zulip,andersk/zulip,showell/zulip,hackerkid/zulip,andersk/zulip,brainwane/zulip,showell/zulip,kou/zulip,punchagan/zulip,andersk/zulip,zulip/zulip,eeshangarg/zulip,andersk/zulip,andersk/zulip,zulip/zulip,eeshangarg/zulip,eeshangarg/zulip,brainwane/zulip,punchagan/zulip,zulip/zulip,zulip/zulip,showell/zulip,brainwane/zulip,kou/zulip,punchagan/zulip,rht/zulip,hackerkid/zulip,showell/zulip,punchagan/zulip,rht/zulip,andersk/zulip,brainwane/zulip,showell/zulip,rht/zulip,punchagan/zulip,eeshangarg/zulip,kou/zulip,zulip/zulip,brainwane/zulip,showell/zulip,hackerkid/zulip,punchagan/zulip,hackerkid/zulip,zulip/zulip,kou/zulip,hackerkid/zulip,rht/zulip