Show and resize user avatars in Zulip search results

[edward-ly]

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Feature request

Which Nextcloud Version are you currently using: master

Describe the solution you'd like
As a follow-up to #33, it would be nice if the default icons could be replaced with the avatar of Zulip user whose message is displayed in the search result.

Is your feature request related to a problem? Please describe.
Currently, such images are not resized correctly for each search result entry.

Describe alternatives you've considered
N/A

Additional context
See the commented out lines in lib/Search/ZulipSearchMessagesProvider.php::getThumbnailUrl which enable user avatars (albeit with incorrect sizes).