Media Support

There's a couple last items I want to check off before this goes in:

- [x] Limit actual file uploading and file insertion to repos you have write access to.
- [x] If an assets directory is not available, upload media the path the post is in.
- [x] Allow the option to alter the file destination of a pre upload when the dialog is open.

But right now, things are in a good place to test! this commit provides

- Link dialog functionality #377
- Drag and Drop support for images on to the page
- File selection for uploading
- A media dialog dropdown in the toolbar #404 
- Assign an `assets` directory in config that displays a listing of available media content
when the media dialog dropdown is open.
- Preview relative media paths
- Some general color contrast changes to the UI