add function to delete downloaded chapters when migrating manga and getFlagsFromPositions fix
(cherry picked from commit 4882896f4d65acabb82771699906b797b3de7662)
# Conflicts:
# app/src/main/java/eu/kanade/tachiyomi/ui/browse/migration/MigrationFlags.kt
# app/src/main/java/eu/kanade/tachiyomi/ui/browse/migration/search/MigrateDialog.kt