Add more detail around the status of a livelesson's startup progress

[Mierdin]

Previously, the Syringe API only provided a single field: Ready, to indicate whether a LiveLesson was indeed ready. This presented a big UX problem

The LiveLesson now has a LiveLessonStatus field, which can be one of three values:

• INITIAL_BOOT
• CONFIGURATION
• READY

This describes the overall lifecycle of a LiveLesson, and allows the front-end to better indicate to the user where the LiveLesson is at in it's progress towards readiness.

In addition to having a Status for the LiveLesson itself, the lesson's endpoints also make use of this same enum, so that the API can inform the front end which endpoints have finished starting. This provides further granularity beyond the INITIAL_BOOT phase, by providing insight into exactly how many endpoints are started, and how many haven't yet.

I elected not to provide per-endpoint status for the configuration phase, mainly to keep the complexity of the change low, and because the time spent in the INITIAL_BOOT phase is likely to be the one keeping folks guessing. Once everything is booted, it's far less likely that the configuration phase will take very long or have problems. However, because the Status enum is agnostic, it could be again re-used for that purpose.

Closes #49