Update TinyMCE Editor

[ovnicraft]

The version used is 3.5.8 as commit updated says edx@b58577a

Last version from TinyMCE is 4.0.11

[cahrens]

This is in-progress under https://github.com/edx/edx-platform/pull/2657. We are working on upgrading to 4.0.16.