Move all answer area content to the bottom of the question area

Summary:
This commit moves the entire #solutionarea div to the bottom of the
question area, unless the question contains an element with the class
name "render-answer-area-here" (in which case the div is placed
immediately after this element).

In addition, this commit removes entirely the old "acceptable formats"
section at the top of the answer area in favor of tooltips attached
directly to individual input elements. This is basically backporting the
approach we've taken in Perseus to khan-exercises. Speaking of which,
this commit also brings the expression editor in khan-exercises more in
line with the Perseus version (rendered output above input) which is a
necessary change because otherwise the tooltip obscures the output.

Test Plan:
Most of these changes were in place during the answer area moving party,
and so in general all exercises should be confirmed to work. Tooltips
were only finished afterwards though, so I've spot-checked the various
answer types myself.

Reviewers: aria

Reviewed By: aria

Subscribers: emily

Differential Revision: http://phabricator.khanacademy.org/D14745
Rearrange answer area content within some exercises

See https://docs.google.com/a/khanacademy.org/spreadsheets/d/1wCONrsK1ujRCLal7bcxS0cVkfpfKiPzapt3SsMJ_zqI/edit#gid=0

Color: Pink

Test Plan:
Take all hints and succesfully answer a question within each problem
type for all modified exercises.

Auditors: alex
Rearrange answer area content within some exercises

See https://docs.google.com/a/khanacademy.org/spreadsheets/d/1wCONrsK1ujRCLal7bcxS0cVkfpfKiPzapt3SsMJ_zqI/edit#gid=0

Color: Red

Test Plan:
Take all hints and succesfully answer a question within each problem
type for all modified exercises.

Auditors: alex
Rearrange answer area content within some exercises

See https://docs.google.com/a/khanacademy.org/spreadsheets/d/1wCONrsK1ujRCLal7bcxS0cVkfpfKiPzapt3SsMJ_zqI/edit#gid=0

Color: Blue

Test Plan:
Take all hints and succesfully answer a question within each problem
type for all modified exercises.

Auditors: alex
Rearrange answer area content within some exercises

See https://docs.google.com/a/khanacademy.org/spreadsheets/d/1wCONrsK1ujRCLal7bcxS0cVkfpfKiPzapt3SsMJ_zqI/edit#gid=0

Color: Gray

Test Plan:
Take all hints and succesfully answer a question within each problem
type for all modified exercises.

Auditors: alex
Rearrange answer area content within some exercises

See https://docs.google.com/a/khanacademy.org/spreadsheets/d/1wCONrsK1ujRCLal7bcxS0cVkfpfKiPzapt3SsMJ_zqI/edit#gid=0

Color: Green

Test Plan:
Take all hints and succesfully answer a question within each problem
type for all modified exercises.

Auditors: alex
Rearrange answer area content within some exercises

See https://docs.google.com/a/khanacademy.org/spreadsheets/d/1wCONrsK1ujRCLal7bcxS0cVkfpfKiPzapt3SsMJ_zqI/edit#gid=0

Color: Cyan

Test Plan:
Take all hints and succesfully answer a question within each problem
type for all modified exercises.

Auditors: eater