Migrate these schemas following the process described in T259163:
(*) Migration for NewcomerTask had already started on T259163 before this task was created. Some patches for it are assigned to that task.
Patches for the creation of schemas:
- NewcomerTask https://gerrit.wikimedia.org/r/c/schemas/event/secondary/+/639256/ [MERGED]
- HomepageVisit https://gerrit.wikimedia.org/r/c/schemas/event/secondary/+/639611 [TO MERGE]