load_models needs to support optional models

[jace]

Consider this use case from Funnel:

@app.route('/<space>/users/new', defaults={'group': None}, endpoint='usergroup_new', methods=['GET', 'POST'])
@app.route('/<space>/users/<group>/edit', methods=['GET', 'POST'])
@lastuser.requires_login
@load_model(ProposalSpace, {'name': 'space'}, 'space', kwargs=True,
    permission=('new-usergroup', 'siteadmin'), addlperms=lastuser.permissions)
def usergroup_edit(space, kwargs):
    group = kwargs.get('group')
    if group is not None:
        usergroup = UserGroup.query.filter_by(name=group, proposal_space=space).first_or_404()

usergroup is being queried manually to support the /new endpoint. The @load_models decorator needs a way to be told that querying UserGroup is optional based on the value of the group parameter. At the moment the None value for group will be passed on to the query.

One way to handle this is for coaster.views to define a singleton (perhaps named OPTIONALQ) that can be passed as the default value in the route. If this singleton is received by load_models, it can skip the query. The singleton can also evaluate to boolean false to support typical use cases.

[jace]

We're discontinuing load_models in favour of custom loaders in ClassView (#167).