Pending reason to metadata

Done in this PR:

- [x] Save metadata to transaction transition
- [x] Pass the metadata to controllers/views
- [x] Make status methods aware of the role of the person who's viewing (author / starter)
- [x] Add translations
- [ ] Reason: `intl`
- [ ] Reason: `verify`

TODO (in the next PR):

- [ ] Conversation status without `__transaction_model`
- [ ] Reason: `other`
- [ ] Log and automatically reject if reason is other

```ruby
TransitionMeta = EntityUtils.define_builder(
  [:pending_reason, :string],
  [:braintree_crash_code, :fixnum])
```