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)
  • 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:

/* 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;