Use `bbb-meeting-size-hint` for load calculation

[defnull]

There is an old proposal for a meta attribute called meta_bbb-meeting-size-hint that can be set by front-ends if the expected meeting size is known (e.g. for classes with a known size), and used by load-balancers to better distribute meetings.

If provided, this value should be used instead of LOAD_PENALTY. The name of the meta attribute is not standardized yet, so it should be configurable.