content: Use directional layout for message media

[yash-agarwa-l]

This fixes the layout of images and video thumbnails in the message list for RTL languages.

Previously, these media containers were hardcoded to align to the Left side with padding on the Right, even in RTL mode. Now, they correctly flip to the Right side (the "start" of the message) with padding on the correct side.

This addresses the second RTL issue identified by @gnprice in this comment.

Before

LTR

RTL

After

LTR

RTL

Since the Difference is very minute, this might help.

[gnprice]

Thanks!

Quick format comment: please avoid @-mentions in commit messages. See this section of our Git style guide:
https://zulip.readthedocs.io/en/latest/contributing/commit-discipline.html#mentioning-other-contributors

The description there is actually a bit forward-looking — GitHub's current behavior, and until next month, is that @-mentions do actually cause notifications. That ends up being spammy, because each time the commit gets rebased (and the rebased version pushed to GitHub), that's a new commit and causes a new notification in the person's inbox.

[yash-agarwa-l]

Thanks for the clarification!
I understand, I've updated the current commit and will be careful about that in the future.

[chrisbobbe]

Thanks, this looks great! Marked for Greg's review.

[gnprice]

Thanks! Looks good; merging.