(bug 43996) Add support for RC_EXTERNAL type in api

- this is for recentchanges
- can be used to filter changes injected from external sources,
such as wikidata and other sources that use this type
Change-Id: I8bd19a8a80b0422ab01d21da1702fc090ffa27d2
Merge "(bug 43996) Add support for RC_EXTERNAL type in api"