Add factory-pattern Object loaders for Admin Views

[miketheman]

For admin routes, we're often a little less guarded, and can lead to 5xx errors in production.

An improvement could be to have a factory pattern on the routes, which pre-hydrates the object from the DB before entering the view, like we do for Users:

warehouse/warehouse/admin/routes.py

Lines 114 to 120 in 5845055

	config.add_route(
	"admin.user.detail",
	"/admin/users/{username}/",
	domain=warehouse,
	factory="warehouse.accounts.models:UserFactory",
	traverse="/{username}",
	)

warehouse/warehouse/accounts/models.py

Lines 52 to 60 in 837de24

	class UserFactory:
	def __init__(self, request):
	self.request = request
	def __getitem__(self, username):
	try:
	return self.request.db.query(User).filter(User.username == username).one()
	except NoResultFound:
	raise KeyError from None

Originally posted by @miketheman in #19695 (comment)

[miketheman]

This can save making existence checks inline in a view, so:

@view_config(
    ...
)
def some_view(request):
    obj_id = request.matchdict.get("obj_id")
    obj = request.db.get(ObjType, obj_id)

    if obj is None:
        raise HTTPNotFound
    do_stuff()

becomes:

@view_config(
    ...
	context=ObjType
)
def some_view(obj, request):
    do_stuff()

Refs: https://docs.pylonsproject.org/projects/pyramid/en/latest/narr/traversal.html
Refs: https://docs.pylonsproject.org/projects/pyramid/en/latest/narr/urldispatch.html#route-factories