Feature/improve media progress

Resolves issues around cursor movement after inserting image.

Fix wordpress-mobile/WordPress-iOS-Editor#394
Fix wordpress-mobile/WordPress-iOS-Editor#301
Fix wordpress-mobile/WordPress-iOS-Editor#132
Also it seems the leading `hiddenChar` is set in one pleace and the trailing `hiddenChar` in another, which could make it difficult to track where it's being added.

I'd remove that from the previous code and add a final line of code along the lines of:

`html = hiddenChar + html + hiddenChar;`

I think it would help on readability / maintainability.