Make number-line only warn about ticks when isTickCtrl is true

Summary: Make sure the user only gets a number-of-divisions warning when the user can set the number-of-ticks value. (Fixes T3245)

Test Plan: Check out this [example item](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22%2A%2ADrag%20the%20point%20to%20label%20%24%5C%5Cpink%7B945%7D%24%20on%20the%20number%20line.%2A%2A%5Cn%5Cn%5B%5B%E2%98%83%20number-line%201%5D%5D%5Cn%5Cn%5Cn%2A%2AWhat%20is%20%24%5C%5Cpink%7B%5C%5Ctext%7B945%7D%7D%24%20rounded%20to%20the%20%2Anearest%20ten%2A%3F%2A%2A%20%20%20%5Cn%5B%5B%E2%98%83%20input-number%201%5D%5D%5Cn%5Cn%22%2C%22images%22%3A%7B%22https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2F159d63e7448bacb403a6ae6aee83b5c36411e13a.png%22%3A%7B%22width%22%3A460%2C%22height%22%3A80%7D%7D%2C%22widgets%22%3A%7B%22number-line%201%22%3A%7B%22type%22%3A%22number-line%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22range%22%3A%5B930%2C952%5D%2C%22labelRange%22%3A%5B0%2C0%5D%2C%22divisionRange%22%3A%5B1%2C10%5D%2C%22labelStyle%22%3A%22decimal%22%2C%22labelTicks%22%3Atrue%2C%22numDivisions%22%3Anull%2C%22tickStep%22%3A2%2C%22snapDivisions%22%3A2%2C%22correctRel%22%3A%22eq%22%2C%22correctX%22%3A945%2C%22initialX%22%3A930%2C%22isTickCtrl%22%3Afalse%7D%2C%22version%22%3A%7B%22major%22%3A0%2C%22minor%22%3A0%7D%7D%2C%22input-number%201%22%3A%7B%22type%22%3A%22input-number%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22value%22%3A950%2C%22simplify%22%3A%22required%22%2C%22size%22%3A%22normal%22%2C%22inexact%22%3Afalse%2C%22maxError%22%3A0.1%2C%22answerType%22%3A%22number%22%7D%2C%22version%22%3A%7B%22major%22%3A0%2C%22minor%22%3A0%7D%7D%7D%7D%2C%22answerArea%22%3A%7B%22type%22%3A%22multiple%22%2C%22options%22%3A%7B%22content%22%3A%22Drag%20the%20point%20and%20fill%20in%20the%20blank.%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%22calculator%22%3Afalse%7D%2C%22itemDataVersion%22%3A%7B%22major%22%3A0%2C%22minor%22%3A1%7D%2C%22hints%22%3A%5B%7B%22content%22%3A%22%24%5C%5Cpink%7B945%7D%24%20is%20between%20%24944%24%20and%20%24946%24%20on%20the%20number%20line.%5Cn%5Cn%21%5B%5D%28https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2Fee472fa29867e1bbfd353d763534f7e7d0b600e0.png%29%22%2C%22images%22%3A%7B%22https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2Fee472fa29867e1bbfd353d763534f7e7d0b600e0.png%22%3A%7B%22width%22%3A460%2C%22height%22%3A80%7D%7D%2C%22widgets%22%3A%7B%7D%7D%2C%7B%22content%22%3A%22Now%2C%20let%27s%20look%20at%20the%20%24%5C%5Cblue%7B%5C%5Ctext%7Btens%7D%7D%24.%20%20%20%5Cn%5Cn%24%5C%5Cpink%7B945%7D%24%20is%20the%20same%20distance%20from%20%24%5C%5Cblue%7B940%7D%24%20and%20%24%5C%5Cblue%7B950%7D%24.%20%20When%20a%20number%20is%20the%20same%20distance%20from%20two%20tens%2C%20we%20round%20up.%20%20%20So%2C%20%24%5C%5Cpink%7B945%7D%24%20rounds%20up%20to%20%24%5C%5Cblue%7B950%7D%24.%5Cn%21%5B%5D%28https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2F4dce36e4ae0940779e792c77e3a6da103b946543.png%29%22%2C%22images%22%3A%7B%22https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2F4dce36e4ae0940779e792c77e3a6da103b946543.png%22%3A%7B%22width%22%3A460%2C%22height%22%3A80%7D%7D%2C%22widgets%22%3A%7B%7D%7D%2C%7B%22content%22%3A%22%21%5B%5D%28https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2Fee472fa29867e1bbfd353d763534f7e7d0b600e0.png%29%5Cn%5Cn%24%5C%5Cpink%7B945%7D%24%20rounded%20to%20the%20nearest%20ten%20is%20%24%5C%5Cblue%7B950%7D%24.%22%2C%22images%22%3A%7B%22https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2Fee472fa29867e1bbfd353d763534f7e7d0b600e0.png%22%3A%7B%22width%22%3A460%2C%22height%22%3A80%7D%7D%2C%22widgets%22%3A%7B%7D%7D%5D%7D) and notice that the number of ticks is > 10 but the user sees no answer AND is able to submit their answer.

Reviewers: aria

Reviewed By: aria

Maniphest Tasks: T3245

Differential Revision: http://phabricator.khanacademy.org/D13636