More robust displaycondition checking

[jarekwg]

Initially the aim of this PR was going to be to broaden displaycondition logic to allow checking against attributes on related models, as well as M2Ms. However I noticed this'd come at a performance cost that probs isn't worth it..
So instead, this PR:

• increases max length of DisplayCondition.values, to allow a larger amount of FKs to be thrown in there.. (not beautiful, but more performant)
• increases explicitness in the code around which kind of fields are allowed to be checked by displayconditions.
• squelches some errors that could've been coming out from the DisplayCondition logic previously, when bad conditions were provided.