Make use of thumbnails

[alya]

In https://github.com/zulip/zulip/issues/16210, we're implementing thumbnailing for uploaded images.

We should use thumbnails in the mobile app. In particular, the message view should show thumbnails, while the lightbox should download the full-scale images.

Blockers:

• https://github.com/zulip/zulip/issues/16210

Flutter issue:

• https://github.com/zulip/zulip-flutter/issues/799

#api design threads:

• https://chat.zulip.org/#narrow/stream/378-api-design/topic/thumbnailing.20transition.20for.209.2E0/near/1886620
• https://chat.zulip.org/#narrow/stream/378-api-design/topic/thumbnailing.20URLs/near/1843871
• https://chat.zulip.org/#narrow/stream/378-api-design/topic/thumbnail.20placeholder/near/1833796

[gnprice]

The current state of this is (release-management chat thread, and see also the #api design threads linked above):

• The server side might be ready to test on chat.zulip.org later today; if not, then hopefully by tomorrow at latest.
• We'll verify nothing goes horribly wrong already with existing clients; if it does, figure out on #api design how to deal.
• We'll verify that once there is a thumbnail, the existing client behaves well and there's nothing to do.
• We'll implement styling for the thumbnail placeholder.

[gnprice]

• We'll verify nothing goes horribly wrong already with existing clients; if it does, figure out on #api design how to deal.
• We'll verify that once there is a thumbnail, the existing client behaves well and there's nothing to do.

Confirmed!

• We'll implement styling for the thumbnail placeholder.

Sent this as #5880.

Details in chat thread: https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/thumbnails/near/1892137