Start integer sum exercise
Working integer sum exercise
Implemented some suggestions
Improved final hint
Add case-insensitive flag to regex answer type
Get rid of unused currExerciseFilename variable.

Test Plan:
1. Load /exercises/parallel_lines_1.html
2. Confirm it still works

Reviewers: alpert

Reviewed By: alpert

CC: eater, emily

Differential Revision: http://phabricator.khanacademy.org/D948
Update khan-site.css

Test plan: Load an exercise in local mode, note that it looks reasonable and
there's no longer a 404 when fetching the logo

Auditors: alpert
Fix #35037

Test plan: Locally test the problem in the issue and the next problem (where
the parabola opens up)

Auditors: stephanie
Rewrite absolute_value_equations to not use multiple choice

Summary: CCing Emily since you might appreciate the interesting answer type hack

Test Plan: Tested locally

Reviewers: stephanie

Reviewed By: stephanie

CC: emily

Differential Revision: http://phabricator.khanacademy.org/D958
Realign linear_equations exercises to tutorial

Summary:
one_step_equations is now only a+x=b
linear_equations_1 is now only ax=b and x/a=b

Test Plan: Tested locally

Reviewers: stephanie

Reviewed By: stephanie

Differential Revision: http://phabricator.khanacademy.org/D911
Fix #35071 minor error in hint

Test plan: Tested locally

Auditors: stephanie
Fix wording for tax problems
Merge pull request #35236 from xymostech/tax-wording

Fix wording for tax problems
Update README with basic info on using local mode

Test plan: Daily for the last year

Auditors: alpert
Fix #35686 fix typo

Test plan: Use a spell checker?

Auditors: stephanie
Fix #35616 Make cube root larger/legible

Test plan: Local mode

Auditors: alpert
Remove a couple (maybe even 3) pixels of whitespace

Test plan: squinted

Auditors: alpert
Add exercise: Logarithms 1.5

Summary:
Inspired by the comment:

> The exercises should include negative and fractional logarithms as well, e.g. log(8,2) = 1/3 and log(4,1/64) = -3.

Test Plan: ... looked at hints for all the different cases? Not sure what one writes in a test plan for new exercises.

Reviewers: eater

Reviewed By: eater

Differential Revision: http://phabricator.khanacademy.org/D979
Make-better the dividing complex numbers exercise

Summary: Now the answer is always an integer and the hints make more sense when dividing by a single term.

Test Plan: ...looked at the hints?

Reviewers: eater

Reviewed By: eater

Differential Revision: http://phabricator.khanacademy.org/D980
Equations should have equals signs

Test plan: tested locally

Auditors: stephanie