Cleanup: removed unused variable declarations
Cleanup: proper variable scoping, use a more meaningful name
Request specific unit ids instead of pages

The concept of pages is problematic for the editor since we are dealing
with a result set of a dynamic nature that is being changed in the server.
This resulted in unexpected behaviors and reliability was missing when
performing certain actions in the editor.

From now on the client will request an explicit list of unit ids the
server needs to query and return. The client keeps a sorted list of
unit ids fetched in the very first request, therefore it knows which units
it has and which ones are missing.

Fixes #48.
Cleanup: align naming
Cleanup: renamed `UnitCollection` to `UnitSet`
Identify 404 errors as such
Cleanup: moved result set bits to the unit collection
Cleanup: use a more sensible name
What's the point of passing unnecessary data?
Cleanup: minor coding style nit
Simplified the way to set the active unit