Categorize events internally by size

[Igor Santos]

Would be nice if the event could be categorized by small/medium/large size.

This could be guessed by asking the expected number of participants, or by using a direct select dropbox (with expected number of participants as examples so the user can understand what we consider small/medium/large).

The sizes could be:

• Local: < 75
• Small: < 150
• Medium: < 500
• Large: > 500

This information could be very useful for filtering the search results, and for selecting notification types later on (#94).