Add plain-text handling for rich-text topics as per MSC3765

[Johennes]

This implements matrix-org/matrix-spec-proposals#3765 which is already merged and, therefore, can use stable identifiers.

For /publicRooms and /hierarchy, the topic is read from the eponymous field of the current_state_events table. Rather than introduce further columns in this table, I changed the insertion / update logic to write the plain-text topic from the rich topic into the existing field. This will not take effect for existing rooms unless their topic is changed. However, existing rooms shouldn't have rich topics to begin with.

Similarly, for server-side search, I changed the insertion logic of the event_search table to prefer the value from the rich topic. Again, existing events shouldn't have rich topics and, therefore, don't need to be migrated in the table.

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