Internalize the Navis Media Credit plugin

It's a required plugin for the theme, and will make it easier for us to maintain and push updates
Fix issues with PHP notices and sanitizing in the media credit plugin
Resource needs to be loaded relative to the theme
Don't use the Navis Media Credit TinyMCE plugin for TinyMCE 4

It's broken and not easy to fix
Correct the use of `version_compare()`
Merge pull request #213 from INN/fix-tinymce

Incorporate Navis Media Credit into the theme