Fix some codeclimate findings

fix some code climate warnings

This is a part of https://www.openproject.org/work_packages/2664
(though, the mentioned work_package needs some more pull requests, so a single PR does not grow too much)

A changelog-entry may be created with the last PR of that work_package.

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fixes 13 mass assignment issues (none of them should be critical)
makes ApplicationController and CustomField more complex (they are still F rated)
improves CustomFieldController (C -> B)
see: https://codeclimate.com/repos/52330269f3ea006d790604fa/compare/fix%2Fcodeclimate_findings