Add support for series

[slimndap]

Makes it possible to group several events in a serie (eg. a festival).

Possible approach:

• An event can belong to one or more series.
• A serie has the same fields as events (title, description, thumbnail, categories), but does not have event dates.
• A serie can show up in lists, based on the dates of the child events.
• A serie has a details page, just like events.
• The details page of a serie can show a list of child events (optionally grouped by day, month, year or category).
• tbd...

See #137 for a similar request.