Don't use strong params where not necessary

* Just use params when no updating is happening
Rationalise strong params for mappings

* Use vanilla `params` when they're not to be used in a mass assignment
* Delete things that aren't used in mass assignment
* Split mapping_params and mappings_batch_params in two
* BulkEditor copies params that it uses in mass assignments, so just
  give it vanilla `params`
Remove redundant strong params

* Weren't being used in any mass assignment