Show message when no metadata is present

[lmarini]

For both User Metadata and Machine Metadata show some useful text when there is no metadata yet. Something that explains that what user metadata and machine metadata are. For example:

"Currently there is no user metadata provided for this dataset. To start adding some click on the Add metadata button. User metadata is metadata about the dataset added by any user who has write permission to the dataset. The list of available field is defined by the administrators of the system. If you would like to add a new entry please contact one of the administrators.