Refactor sequence track.

Apart from cosmetic changes to code,
* Previously there was a gap b/w coloured box around M and *, and it's parent
  div. Removed that.
* Assign proper CSS classes to all sequence divs. Primarily, this makes
  debugging easy. The intent is not to style sequence track any further.
* Use `display: table` for sequences. This simplifies the code a lot.

Signed-off-by: Anurag Priyam <anurag08priyam@gmail.com>
Change ordering of frames in sequence track.

Now,
  D     frame 3
 *      frame 2
M       frame 1
ATGACG  forward
TACTGC  reverse
     R  frame -1
    V   frame -2
  S     frame -3

Before (WA style),
  D     frame 3
 *      frame 2
M       frame 1
ATGACG  forward
TACTGC  reverse
  S     frame -3
   V    frame -2
    R   frame -1

Signed-off-by: Anurag Priyam <anurag08priyam@gmail.com>
Allow downloading FASTA

Signed-off-by: Anurag Priyam <anurag08priyam@gmail.com>