New feature documentation - migration of attachments and media files to content hub

[tkrch]

PR #243 adds ability to migrate all media files and attachments to content hub.

this feature is activated in appsettings.json with following setting.
"MigrateMediaToMediaLibrary": false,

this will be default setting after documentation and further tests.

Feature description

Media libraries are migrated into content folder named "[SiteName]/[LibraryFolder]". All of them are migrated only to default language to avoid unwanted replication of file binaries over all language versions (Media library was language agnostic)

Attachments are migrated into content folder named "[SiteName]/__Attachments". They respect language version when language version is known (for example when attachment is use on document)

References to media file & attachments

references are migrated to content item references anywhere where we know about them (Content item types, widgets). There is option to define this migration explicitly for field and form control in following way (in appsettings.json section Settings:OptInFeatures:CustomMigration:FieldMigrations):

{
  "SourceDataType": "text",
  "TargetDataType": "contentitemreference",
  "SourceFormControl": "MediaSelectionControl",
  "TargetFormComponent": "Kentico.Administration.ContentItemSelector",
  "Actions": [
    "convert to asset"
  ],
  "FieldNameRegex": ".*"
}

Possible customization of feature

the manner in which are entites converted and store can be customized inside class Migration.Toolkit.Source.Services.AssetFacade