Solve Bug with long texts and media

[TamaroWalter]

Pull Request for Issue #8

What was the problem?

• The postletter.js cuts too long text and makes the original text appear with a show more button. The "cut"-function worked with the inner-HTML of the post-message and because of that it cutted within html-elements, that lead to this problem. In text-only messages this was not a problem, messages with videos and latex-commands it showed.

What is the current solution?

• Instead of working with the message itself the functionality of cutting posts letters at a certain point is solved with css-styling. By restrict the height of the message-div to a certain limit and making it expandable with the "show more"-button, the same functionality is achieved without manipulating html-elements directly.