adding requested exercise: converting fractions and mixed numbers to percents

This was a green card request on Trello. 

Exercise:
https://github.com/zpardos/khan-exercises/blob/learn2/exercises/converting_fractions_and_mixed_numbers_to_percent.html

It uses code from:
converting_mixed_numbers_and_improper_fractions.html
changing_fractions_to_percents.html

History:
3/15: I commented on Trello that I was going to work on this request
3/17: Christi R. on Trello advised me that a fractions->percents pull request had been made by Owen (https://github.com/Khan/khan-exercises/pull/15495.) so I decided to work only on the mixed number -> percents problem
3/18: Christi discussed with me in hipchat that she wasn't sure how long ago Owen had worked on fractions->percents and advised me to do both fractions and mixed numbsers -> percents
3/19: Submitting pull request

Ideas:
Sal's video on percents emphasized the meaning of "percent" to be equivalent to "per 100". I think matching a hint strategy up with this idea would be of benefit to students (depending on pre-requisite mastery). An example would be to treat the conversion from fraction to percent as an algebra problem where the goal is to multiply the numerator and denominator by X such that the denominator becomes 100. The numerator then would equal the percent answer.