Bug fixes and improvements of translation UI

- Fixed copying original translations on the „updated translations“ page.
- Copying of the original translation is now done via a hidden, disabled textarea in order to preserve line breaks.
- Cleaned up the views a bit.
- Highlighting of interpolations is done by a Ruby helper method (like highlighting of line breaks) instead of JavaScript.
- Line break marker now uses the ¶ symbol, as is common in Rich Text Editors.
- The ¶ symbol is being added using a CSS :before filter. With that Mozilla and Webkit-based browsers do not include the symbol when the user manually selects the text for copying&pasting which is a good thing.
- Refactored the javascripts for interpolations and actions.