Mandatory fields and help texts in generic events and webinar

• Set “event type” field to be mandatory in generic event
• remove “event type” from Webinar form
• Set image field to be mandatory in Webinar (not only in generic event) because when they are upcoming it is needed in the upcoming events view
• In the field "location" -> Add text "Enter location with the following format: 'City, Country'"
• In the field "image (from the behaviour "ILeadImageBehavior") -> Add text "Enter image to be displayed in the summary view only"
• Add a field that allows to enter a list of images as content type field (now the images are loaded as Plone Image contents in the event's folder)
• Add a field that allows to enter all the presentations

Default field values are assigned to objects when they are first created. A default factory can be specified to dynamically compute default values.