Change ID columns from `bigint` (64-bits) to `integer` (32-bits) on various tables.
Rails 6.0 switched the default from bigint to integer for IDs on new
tables, so now we have a mix of tables with integer IDs and bigint IDs.
Switch back to integer IDs on certain tables because we're going to
build a view that unions a bunch of tables together to build a user
activity timeline, and for this purpose all the tables need to have IDs
of the same type in order for Postgres to optimize the query effectively.