optimise card/deck creation workflow

[blahah]

some simple things to make the process smoother:

• have a 'save + new' button that opens a new blank slide after saving the current one
• add keyboard shortcuts for basic actions like:
  • save
  • save+new
  • switch between slide and metadata
  • add new metadata field
• auto-focus the first metadata field when a slide is opened to create or edit