Merge branch 'flashcards-revisited' into 'master'

Flashcards revisited

These changes introduce several UI & UX  improvements, bugfixes and new features.  

---
##### Major changes:
- New question variant based on flashcards (like lecture and preparation questions)
- Flashcard variant can be added to custom sessions through feature activation
- Improve flashcard presentation in question carousels
   - front- and backside are placed in one visible element ([backface-visibility](http://www.w3schools.com/cssref/css3_pr_backface-visibility.asp))
   - front- and backside are configurable through style.scss
   - flashcard size is determined dynamically based on front- and backside dimensions
- Add smooth flip animation to flashcards (based on CSS3, not sencha)
- Add feature to flip all flashcard in one session (triggered by socket)
- Add import and export to ARSnova.cards   

---
##### Other changes:
- visibility of speakers overlay buttons is not longer bound to projector mode. New condition: screen width > 840px
- configuration options for question carousels inner item width have been added to style.scss
- all labels and buttons in AudienceQuestionPanel and QuestionDetailsPanel have been customized for flashcards
- a new list button for new question variant "flashcard" has been added to both inclass panels
- question import and export modals have been completely overhauled  for a consistent look and feel
- several UI & UX improvements    

---
##### Bugfixes:
- CSV import has been fixed: Import failed if rateable questions without correct answers were inside CSV-file
- **critical:** fix layout problems in latest firefox release caused by senchas flexbox implementation   
- Question and answer preview have been reintroduced for rateable question formats
- resolve several javascript reference errors   

  ---
###### New configuration options:  
```sass
/* carousel inner item width configuration */
$carousel-inner-max-width: 80%;  // was 720px previously

/** flashcard front side configuration **/
$flashcard-front-font-color: $brand-text;
$flashcard-front-background: linear-gradient(to top, #DFE8EC 0%, white 5%) 0 20px;
$flashcard-front-background-size: 100% 30px;
$flashcard-front-border-width: 5px;
$flashcard-front-border-color: white;
$flashcard-front-border-style: solid;

/** flashcard back side configuration **/
$flashcard-back-font-color: $brand-text;
$flashcard-back-background: linear-gradient(to top, #DFE8EC 0%, white 5%) 0 20px;
$flashcard-back-background-size: 100% 30px;
$flashcard-back-border-width: 5px;
$flashcard-back-border-color: white;
$flashcard-back-border-style: solid;
```

See merge request !39