Fix migrations that use enable_extension with table_name_prefix/suffix

When using `ActiveRecord::Base.table_name_prefix` and/or `table_name_suffix`, extension names get the same treatment as table names when running migrations. This leads to migrations that try to call, for example, `enable_extension("prefix_hstore_suffix")` on the connection.

This pull request fixes that and adds some tests for both `enable_extension` and `disable_extension` in migrations. I think it should also be backported to _4-0-stable_. I can send another pull request for the backport if needed.