Add missing metadata fields to "Create Event" process

[mtneug]

With #691, the Start date field moves to the metadata step. However, this step is still missing some metadata fields. Compare view for the event and creation dialogues:

• Start date is added by All metadata should be in the metadata step of the add event wizard #691 for upload events
• Duration is missing and could also be added for upload events
• Location is missing and could also be added for upload events
• Source is missing and could also be added for all events
• Created and UID are set automatically