build a smaller/faster temporary index for FF migration

we've had at least 2 people on Discord migrating from FF and getting errors like:

ERROR:  index row size 3496 exceeds btree version 4 maximum 2704 for index "IDX_51c063b6a133a9cb87145450f5"

presumably because too make attachments to some note

since these indexes will be dropped by our TypeORM migrations, we don't really need to build them the same way as FF; let's make this one smaller and faster