add unique indexes to tables columns that require unique values

https://osraav.atlassian.net/browse/OSRA-252
OSRA-252 #code-review
I don't know what to think about this. On the one hand, this is the only way to absolutely ensure uniqueness. On the other, these are models that will change practically never *and* there is no UI, for now at least, for the end user to be able to change them anyway. The latter part of this means that there is no way a near-simultaneous save to the db can occur. Since there are some costs associated with indexes, are we hurting performance needlessly by adding them at this stage?

On a related note, I just ran [this script](https://tomafro.net/2009/09/quickly-list-missing-foreign-key-indexes) on our app - `pending_orphan_list_id` is not indexed in pending_orphans table; province_id, orphan_original_address_id & orphan_current_address_id` are not indexed in addresses; `sequential_id` is not indexed in partners, sponsors & orphan_lists tables; `orphan_sponsorship_status_id` is not indexed in orphans. The last 2 are particularly bad - I'll submit a PR shortly.