Removed limit of 25 groups and past mailings

[larssandergreen]

Before: Saving a mailing with more than 25 groups or past mailing recipients caused the number of groups to be reduced to 25.
After: As many groups as desired can be added.

[civibot]

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[eileenmcnaughton]

sigh another one - we removed this default in apiv4

[totten]

Looks pretty sensible 👍