UIHints for block and media folders should be merged into an assets folder

Description

There are currently two UI hints to be able to select a folder: BlockFolder and MediaFolder. These should be replaced with a generic mediafolder since the structures are now merged. A guess is that folders might appear in other repositories in the future, so it's probably good to name it AssetsFolder instead of just Folder.

Note: This requires changes in both the EPiServer assembly as well as in the CMS UI. Since the connection between these is just a string, the UI can use a hard coded string until it can depend on an updated EPiServer assembly.

See related core bug, requires sync. The UI implementation should be done before core changes to avoid shipping unusable UI hint.