Store `SiteSlurmQOS` limits as a collection of Allocations

[camillescottatwork]

Currently, SiteSlurmQOS is defined like so:

class SiteSlurmQOS(BaseDocument):
    sitename = StringField(required=True)
    qosname = StringField(required=True, unique_with='sitename')
    group_limits = EmbeddedDocumentField(SlurmTRES, default=lambda: SlurmTRES())
    user_limits = EmbeddedDocumentField(SlurmTRES, default=lambda: SlurmTRES())
    job_limits = EmbeddedDocumentField(SlurmTRES, default=lambda: SlurmTRES())
    priority = IntField()
    flags = ListField(SlurmQOSFlagField())

We should define a new object SlurmAllocation that encapsulates a SlurmTRES along with a creation_date and expiry_date, and then define the _limits fields as collections of these. This would allow us to auto-expire portions of the QOS and keep us from having to track these expirations externally.