Add migration to add index to medias.file_path for SQLite3

migrations/sqlite/2021-02-23-153402_medias_index_file_path/down.sql

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+DROP INDEX medias_index_file_path;

migrations/sqlite/2021-02-23-153402_medias_index_file_path/up.sql

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+CREATE INDEX medias_index_file_path ON medias (file_path);