New permission settings are ignored, if many forums are edited at once

If a lot of forum permissions are set all at once (e.g. by ckecking "all forums" box on forum selection), the changes that are made to the permissions are ignored upon submission. Instead one is returned to the form, where the new permissions are supposed to be entered, none of the changes are stored.

Possibly related to https://tracker.phpbb.com/browse/PHPBB3-14315

We needed at least 30 Forums to reproduce this error, potentially even more than that.

See also https://www.phpbb.de/community/viewtopic.php?p=1349155#p1349155 (german)

Steps to reproduce (wording may differ as I did this with a translated board):

• Ensure there is at least 35 - 40 forums existent in the test board
• Navigate to Permissions -> Forum based permissions
• check the "select all forums" box and submit
• select an arbitrary group to edit permissions (there were already permissions present for that group so it was listed in the top right of the form)
• Change the permission roles for some arbitrary forums
• Submit
  You should now be returned to the exact same form, except no permissions were changed. No confirmation message is shown.