Hygiene: Rename SchemaPopupsSamplingRate
The SchemaPopupsSamplingRate config variable is inconsistently named and
even forwarded to the client as wgPopupsSchemaPopupsSamplingRate.
Changes:
- SchemaPopupsSamplingRate -> PopupsSchemaSamplingRate
- Forward PopupsSchemaSamplingRate to the client as $wgPopupsSchemaSamplingRate.
Bug: T146889
Bug: T146434
Change-Id: I80d6a0abccf462e2eb0fd96af6849b5e82b49c26