Add a "Add component" button

[slamkajs]

Create a backbone view to display a button which opens a dropdown with a list of components (organized by type)

when the component is clicked on, a dialog should open with a form to gather necessary info for component.

Dialog forms

• see issue Create assessment component add/edit view #69: assessment component
• see issue Create audio/file download/Document Viewer/video component add/edit view #70: audio/file download/Document Viewer/video components
• see issue Create External Content component add/edit view #71: External Content component