Fixes tinymce bug:
    On using the image plugin, the image was not appearing in the
    visual editor immediately after using the command.

    Selenium test added.
Slightly reorder the toolbar buttons so that the image button is more likely to be visible.

Hopefully we will get a better solution with the TinyMCE upgrade PR.
Merge pull request #2737 from edx/dcadams/fix_tinymce

Fixes tinymce bug: