change attachment location col to type :text

can confirm that the migration preserves existing attachments (click links on clone)
 
also tested this locally and on clone with md5 hashes:
```ruby
Digest::MD5.hexdigest(Attachment.all.map(&:location).join(''))
```

### Local :white_check_mark: 
> before_migration 599085c43db994c56e365e3eb6ad4989
> after_migration  599085c43db994c56e365e3eb6ad4989

### Clone :white_check_mark: 
> before_migration 352d2fa09d9fdbaf56ad825ddc8b582b
> after_migration  352d2fa09d9fdbaf56ad825ddc8b582b