move i18n of UserActions to JS-side and make sentences translatable (take 3)

Follow up of #677 and #689. Now the strings look like this:

    you_replied_to_topic: "<a href='{{userUrl}}'>You</a> replied to <a href='{{topicUrl}}'>the topic</a>"
    user_mentioned_user: "<a href='{{user1Url}}'>{{user}}</a> mentioned <a href='{{user2Url}}'>{{another_user}}</a>"