UI to manage series of events

[pferreir]

We are already "silently" linking together events whenever someone creates a lecture series. This is the concept of Series, which we introduced in 2.0.

However, we don't have a UI where organizers can manage their series, so it's impossible to e.g. add an additional lecture to a series without an admin using the CLI. The idea would be to implement a basic management UI with the following features:

• Creating/managing an event series (just a "group of events");
• (Un)linking an event to/from an existing series;

See also #2993. (out of the scope of this issue, much more complex and should NOT be done at the same time because of this)

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Main tasks to do here:

• Add UI for users to create a new series
• Add UI for users to manage a series (edit its settings and add/remove events)
• Add new option to include new event(s) to the series while cloning if the current event is part of a series
• Create an event selection widget for the series UI - either pasting links to the event, or a search field that searches by title within the same category. Could also suggest events straight away (based on title/dates). To be discussed w/ the Indico team (and possibly mocked up)