Handle case in combining_like_terms_2 where constant terms simplify to 0

Summary:
fix #26874; fix #29882; fix #29943; fix #29949; fix #29964; fix #29969;
fix #30088; fix #30091; fix #30108; fix #30118; fix #30138; fix #30177;
fix #30205; fix #30257; fix #30258; fix #30276; fix #30279; fix #30280

Previously, the last hint would show something like "-24a + 0" and the regex
answer would expect you to type "-24a0". This fixes the final hint to show just
"-24a" and the answer regex expects "-24a"

Test Plan:
Tested these problematic seeds outside devappserver (before and after the fix):
http://localhost:8000/exercises/combining_like_terms_2.html?debug&seed=112&problem=c_fd_fbx
http://localhost:8000/exercises/combining_like_terms_2.html?debug&seed=177&problem=c_fd_fbx
http://localhost:8000/exercises/combining_like_terms_2.html?debug&seed=161&problem=c_fd_fbx

Reviewers: stephanie

Reviewed By: stephanie

Differential Revision: http://phabricator.khanacademy.org/D742
Fix up exercises that are too wide for tutorials

Reviewers: marcos, tom, desmond

Differential Revision: http://phabricator.khanacademy.org/D737
Add calculator

Test Plan: Tested outside devappserver by typing in some random expressions

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D756
Fix calculator exponent associative order
Merge pull request #30647 from xymostech/fix-calculator-exponent

Fix calculator exponent associative order
Support positioning of graphie labels by specifying an angle

Test Plan:
Created a dummy exercise that plotted a point and drew a label at
various different angles relative to the plotted point.

Reviewers: eater

Reviewed By: eater

Differential Revision: http://phabricator.khanacademy.org/D766
Add exercise: Linear equation word problems

Summary:
These exercises were made a while ago but were never used. Now, they should be
good!

Differential Revision: http://phabricator.khanacademy.org/D498
Add exercise: Graphs of inequalities

Summary:
new problem type in graphing_inequalities

finished hints sans link

fixed inequality hints

added more hints for point-slope formula in inequalities

removed transformations exercises...

Differential Revision: http://phabricator.khanacademy.org/D433
Don't taunt people with a disappearing calculator
jQuery .data() expects just a key when retrieving values.

The existing code breaks some exercises (like function_1 and many more)
when using a version of jQuery greater than 1.7.1.

See http://api.jquery.com/data/
Merge pull request #30904 from saasmath/master

jQuery .data() expects just a key when retrieving values
Add exercises: Law of sines and Law of cosines

Test Plan: Tested multiple seeds for each new problem type outside devappserver

Reviewers: stephanie, yunfangjuan

Reviewed By: yunfangjuan

Differential Revision: http://phabricator.khanacademy.org/D776
Add exercises: Graphing circles 1 & 2

Test Plan: Tested multiple problem seeds outside devappserver

Reviewers: stephanie, yunfangjuan

Reviewed By: yunfangjuan

Differential Revision: http://phabricator.khanacademy.org/D777
Merge and squash pull request #31164 from saasmath/master

Closes #31164

commit 1261caef7cafbdf46285e3185b89d3fb71601dab
Author: Gary Anderson <git@garyvictoranderson.com>
Date:   Wed Oct 3 04:14:58 2012 -0700

    Accept approximate decimal answers in computing means

commit 08abd643e0df124ecf43600e9ad85077e14c987e
Merge: 87bc7c5 d1f63a4
Author: Gary Anderson <git@garyvictoranderson.com>
Date:   Wed Oct 3 03:55:57 2012 -0700

    Merge branch 'master' of https://github.com/Khan/khan-exercises

commit 87bc7c506f0265cf7aeab36dc6634b7a495f0111
Author: Gary Anderson <git@garyvictoranderson.com>
Date:   Tue Oct 2 03:27:10 2012 -0700

    Accept inexact answers for area computations using z-scores.

    In math and statistics courses, many students are taught to use graphing calculators or other
    technology, rather than tables, to compute areas under the normal curve.  These will differ
    in the last digit frequently from the approximate z-tables, leading to incorrectly reported errors in
    the exercises.
    This fixes the issue by accepting the correctly rounded technology produced answer.
Add matrix addition exercise. Also, add prependVars to tmpl.js.
Remove unused import
Add exercises: Matrix addition and Matrix subtraction

Summary: Also, add prependVars to tmpl.js.

Test Plan: Tested locally.

Reviewers: eater, yunfangjuan

Differential Revision: http://phabricator.khanacademy.org/D797
add scalar matrix multiplication exercise
fixes for D797
refactors for D797