Changed delete conversation button tooltip

Delete conversation button currently says 'delete and block conversation'.

When I click that button I expect to delete or block that conversation from DB so that neither me or any participant can see it or reply.

Currently, signed in user is deleted from visibility table, so he is not longer part of the conversation.

"Leave conversation" is an appropriate text to describe button behavior.

Some localization files are update as well.
```ruby
it 'returns "hidden"' do
```
(the 's' is missing :-P)