Add existing unit workflow (course level)

[sdaitzman]

Modify empty case of new unit at both course overview level and unit page level: add your own component or reference an existing unit page → overlay sidebar, pick library, pick unit

[jmakowski1123]

Minimal implementation:

• Only reuse workflow option is via the course outline page
• From the content outline page, under the "units", add a new button under "add unit" and "paste unit" for "use unit from library"
• Library search modal opens from the course outline page
• Units can be added one at a time (same as the component reuse workflow)