Fixed CodeMirror editor view height in studio.

[TNL-920] (https://openedx.atlassian.net/browse/TNL-920)

```.CodeMirror``` height was less than ```.modal-content``` height, because of this CodeMirror view was getting cut off, fixed by equalising the height.

[Sandbox Studio Link] (http://studio.waheedahmed.m.sandbox.edx.org/)