Merge branch '8.x-3.x' into RestObjects

* 8.x-3.x: (22 commits)
  Couple more changes to adding fields UI. CSS is still garbage, but the new mechanism is working now.
  undefined variable in basefielddefinitions
  by aaronbauman - prevent exception within catch()
  checking off more todos!
  by aaronbauman - remove salesforce_mapping_entity_delete in order to preserve historic record of mappings. This can be useful for data forensics. - final tweaks on push before merging upstream
  Fixes method call in SalesforcePushEvent constructor
  entity and mapping on Push Event
  by aaronbauman - refine PushParams class and events
  Check off that todo
  by aaronbauman - get rid of SOAP-specific fieldsToNull
  Add REST version of SalesforceMapping::getPushParams
  Fixes namespacing errors
  Fixes a lot of syntax errors
  by aaronbauman - more events
  .
  by aaronbauman - initial pass at Events framework. Add event "PushParams"
  by aaronbauman - handful of style tweaks and updates to Constant and Token properties
  by aaronbauman - refactor field mappings to make plugins more pluggable - "validate" delegation - "submit" delegation - all mapping fields are delegated to field plugin system - get rid of tables on field mapping layout - continue thinning out SalesforceMappingFieldsForm controller
  exception handling for pull / mapping loads, etc. ANOTHER BROKEN COMMIT - NEEDS TO BE FIXED STILL
  Walking through field mapping pull loop intent w Rhodes. This commit will probably break things.
  ...

Conflicts:
	modules/salesforce_mapping/src/Entity/MappedObject.php
	modules/salesforce_push/salesforce_push.module
	src/Rest/RestClient.php