Use a priority queue to determine order of MV builds in edda

When streaming patches out to the front end, it would be helpful for the
front end's reactivity to be able to send lists early so it can display
skeletons for individual items it hasn't gotten the patch information
for yet.

Right now, there are two "priorities":
  * Lists
  * Details (everything else)

By default an MV will use the "Details" priority. If the
`build_priority` annotation is provided for the MV struct definition,
then it will use whatever is specified in the annotation.

This also re-structures how MV build tasks are spawned so that we can
get everything queued before letting the priority queue handle things
from there, to ensure the higher-priority items are always built first.
Rather than having to explicitly check if the Change's `entity_kind` is
the same as the MV's `trigger_entity`, this check has been abstracted
away into the `MaterializedViewInventoryItem` and is checked during the
initial queue population. This means that whenever we go to spawn an MV
build task, we already know that the change is relevant for that
specific MV kind.

Author: jhelwig