Add Vuex state for managing the case beginning-to-end

[isms]

Remove view guards (at least for right now) - the plan is just not to show UI for elements based on what is in the global state (e.g. no case yet ⇒ don't show detect and select, no marked candidates yet ⇒ don't show annotate and segment, etc).

[pjbull]

LGTM, go go vuex store!

[reubano]

guess this addresses #264?

[pjbull]

@reubano @lamby More fun to review. This branch also still has a lot to be turned into issues. The goal here was to get most of the API calls into state.js except for on "Open Image", which needs to do a little extra work.

TODOs, which we'll probably turn some of these into issues.

• Refactor the OpenDicom viewer to be more succinct vue code
• Pull out old API style endpoints
• Create in-memory model objects and serialize those rather than calling the DICOM loading methods on the model
• Error handling

[reubano]

@pjbull looking now. btw, I commented on #261, but since it was already merged, not sure if it sent you guys the notification. just fyi..

[isms]