Fix/copy project with required custom fields

[ulferts]

Fixes:

• Uses values of custom fields in source project to fill the fields in the project copy form. That way, the user can choose to keep the values or to alter them. The explicit option to copy the project custom fields has been removed as it was unclear what was to happen when the user sets the custom values via the form while choosing to have them copied. https://community.openproject.com/projects/openproject/work_packages/26795
• Copied work packages will also have the custom values of the source custom field.

Cleans up:

• before action not in use
• cuke that was rewritten as a feature spec
• controller code

[oliverguenther]

LGTM 👍