Migration: splitted mysql database fixes

Migration of a sqlite database to a mysql one didn't create
create_index_if_not_exists() stored procedure in the thumbnail database,
leading to failed update if index already existed.

Data of the thumbnail database is not copyed at all, but the content of
the "Settings" table are needed and checked. Workaround the problem
checking/inserting the two needed records at creation time.

CCBUG: 277242
CCBUG: 276052