Use URI.join to assemble the URL to fetch a single post

That normalizes any extra / on the base URL.
use URI.join in the specs too
BUG: new messages to person that is not sharing

Conversation recipient validated

- Conversation foreign recipient not required to be validated.
- Added 'peter' as spec_helper user.
- New fixture, new statistic values.

- Clearer identifier names
Merge pull request #5449 from margori/3016_block_messages_to_not_sharing_user

BUG: new messages to person that is not sharing