Revisit model field `max_length` properties

[bri25yu]

From anson.tiong:

Was there a reason for this limit? I'd like to give more flavortext for my event, but I'm limited to these 500 characters now

This specific example is for https://github.com/compserv/hknweb/blob/master/hknweb/events/models/event.py#L20, but we should revisit all the current restrictions on max_length on all our models even if this is a pretty extreme scenario.