Handle legacy `!gravatar` images (`img.message_body_gravatar`)

[gnprice]

Up until 2020 (Zulip 3.0), Zulip Markdown had a syntax !gravatar(…), which produced elements like <img class="message_body_gravatar">. We don't currently support this content feature.

Server commit removing the feature: zulip/zulip@912e372
Server commit adding it (2012-10, when Zulip was two months old): zulip/zulip@c31462c
Example message

Although the feature was removed in Zulip 3.0 and we only support much newer servers, existing messages from years ago still have it in their HTML. So we should either support this feature, or get existing instances migrated on the server to modern syntax. The feature isn't a high priority, though.

Found by @PIG208 with the survey #917 / #190.