load_models should support joined loads

[jace]

A typical load_models decoration looks like this:

@load_models(
    (Profile, {'name': 'profile'}, 'g.profile'),
    (ProposalSpace, {'name': 'space', 'profile': 'profile'}, 'space'),
    (TicketType, {'name': 'name', 'proposal_space': 'space'}, 'ticket_type'),
    permission='event-edit')

It asks for instances of Profile, ProposalSpace and TicketType to be loaded in sequence, raising a 404 if one is not found. This requires three database roundtrips, which is inefficient. This can be done in one query if we work up from the bottom:

TicketType.query.join(ProposalSpace).join(Profile).filter(TicketType.name == name, ProposalSpace.name == space, Profile.name == g.profile).first_or_404()

To support this syntax, load_models could change the tuple format it accepts slightly. The second item in the tuple is a mapping of a model attribute to a view parameter, for use with the filter_by filter. This could instead be a column (a SQLAlchemy Instrumented Attribute) that is applied with a filter filter. Since the column-containing models are also required to perform a join, the wrapping tuple could accept a fourth item, a model or a tuple of models that must be joined (L-to-R) (or this could be guessed by looking up the models to which the given filter columns are attached).

[jace]

Caveat: This will break redirect handlers in the chain like this example:

@load_models(
    (Profile, {'name': 'profile'}, 'g.profile'),
    ((ProposalSpace, ProposalSpaceRedirect), {'name': 'space', 'profile': 'profile'}, 'space'),
    (TicketType, {'name': 'name', 'proposal_space': 'space'}, 'ticket_type'),
    permission='event-edit')

[jace]

Joined loads are supported via InstanceLoader introduced in #167. This ticket is now irrelevant. The redirect handler issue can be solved by writing a custom loader wherever it is needed.