feat: stop calling course blocks rest API and assume LS exists (#803)

- Assume that Learning Sequences is available (waffle has been
  removed)
- Stop calling course blocks API, which provided mostly duplicated
  information now.
- Refactor a bit to avoid needing to globally know which units
  exist in sequences. That is now provided just-in-time for only
  the current sequence.
- Add /first and /last URLs that you can use instead of unit IDs
  in URL paths, in service of the above point.

AA-1040
AA-1153

src/courseware/data/index.js

@@ -7,6 +7,7 @@ export {
 } from './thunks';
 export {
   getResumeBlock,
+  getSequenceForUnitDeprecated,
   sendActivationEmail,
 } from './api';
 export {