Context: we would like to associate every event with multiple event types (see list in the task description of T355253; we'll allow at most 2-3 types per event). The campaign_events table already has an unused event_type column added long ago, but that was meant to store a single type, not multiple types. Because there is a fixed list of topics, and the list is quite short, and given DBA recommendations (T387483#10760357), we will rename the column to event_types and change its type from varbinary to integer, so it can be used as a bitfield.
NOTE: when designing the schema change, we need a final decision on whether "other" can be used together with other types or not (assuming that existing events will get the type "other" by default). If the answer is yes, we cannot assign it the value 0 as it wouldn't work in a bitfield.
Acceptance Criteria:
- Define the updated database schema for storing types of event
- Implement a schema change for it
- Create the schema in production and beta (T394509)
