Fix hashtag message in a wrong context

The message “There are no posts with this hashtag” is displayed when hashtag search doesn't return any results. But because processHashtag is not only used for hashtags, but also for viewing mentions, this message was also shown in a wrong context - when user with no mentions opens his/her mentions window. So I fixed that, and the second commit is an options string that was added since last time and not localized yet.