Make it simpler to disable some export formats

Right now, if you want to remove the ODF and RTF formats (for example), you need to override export_modal.vm, and use 'enabled' : false (for example). This is not very easy and it makes it harder than it should be when upgrading XWiki later on.

There are several solutions I can imagine:

• Convert these static UIXs into wiki page UIXs so that users can edit them and set "enabled" to false there.
• Introduce some configuration options in the Office module. Add a OfficeServerConfiguration#getExportFormats) and then, for RTF (for example):

  'enabled': $services.officemanager.isConnected() && $services.officemanager.config.exportFormats.contains('RTF')
  

• Another option would be to transform https://github.com/xwiki/xwiki-platform/blob/ee37ebca0b9c36f2c3d426e5ebe17dddbb4999cb/xwiki-platform-core/xwiki-platform-flamingo/xwiki-platform-flamingo-skin/xwiki-platform-flamingo-skin-resources/src/main/resources/flamingo/export_modal.vm#L57 into a Set and thus users would create a UIX page with "enabled" to false to override the default formats.