Redesign the Export modal

There are a few problems with the current Export modal:

• The export type is known only at the end, when the user clicks on the corresponding export button. This means we cannot customize the tree (document selection) based on the export type. For instance:
  • for some export types it's important to see hidden documents, while for others it's important to not see them
  • the document order is relevant for some export types while for others it doesn't matter
  • for some export types we wan to export only the current document by default, while for other export types we may want to include by default all child pages (and their descendants)
• There are two many export buttons (one for each export type). It doesn't scale if the number of export types is large (e.g. if we want to enable export for multiple office formats, all supported by the Office server)
  • The buttons don't have an icon which would help to quickly identity the export type (most common file types have a recognizable icon)
• The accordion we currently use promotes by default only export types that don't support multi-page export (the rest are hidden / collapsed)