Removed the word "at" in message details timestamp

Rationale for this PR:

> 1. We currently do *not* have a safe method which allows to translate and adapt the timestamp format and "at" string (which separates timestamps in the message details à la "SENT: <date> at <time>"). A wrongly "translated" timestamp currently breaks and crashes TextSecure as discussed in https://github.com/WhisperSystems/TextSecure/pull/1309 , so that it cannot be allowed at the moment.
> 2. The word "at" was not localized, which makes localized (non-English) timestamps looking strange

Therefore I propose *for the moment* - until we sanitize translation inputs for the timestamp string -  to simply drop the "at" word.

Dropping "at" simply avoids the need of translation.