Handling large message/result set sizes in search

When retrieving and displaying a large result set of messages (as in 150*n mb) during search, we need to make sure that:

- We do not exceed the configured http client buffer size (resulting in `entity content is too long [...] for the configured buffer limit [...]` exceptions otherwise, where the default is 100Mb)
- We do not overload the browser during rendering

Potential strategies could include:

- Always retrieve truncated summaries for messages when fetching result sets for multiple messages
- Retrieving complete messages only when showing message details and only on demand (when expanding a message)