Embeds

- Added embedding for Vimeo, Youtube, pdf, or URL links.
- Added 'content' controller to decide which view to use for embeds
- unit-v modified to change link depending on content type
- unit-v modified in case a title was not set for a material (it will
just show the link if a title was not set)
- materials_m modified to include a function to retrieve material info
based on material id (used by content controller)