Show message timestamps on swipe

[gnprice]

With #446 adopting the mobile design from Figma for the layout of messages (as opposed to this client's current design there which was copied from web), we'll lose the timestamps at the right, except on the first message in a row from a given sender, in order to reclaim that horizontal space for the message content.

As a result, as @chrisbobbe explains at https://github.com/zulip/zulip-flutter/pull/446#pullrequestreview-1787600537 , we'll stop showing some useful information up front:

• where the boundaries between a sender's messages are;
• what time they sent each of their messages.

We've had a similar dilemma in zulip-mobile, with the same need to make good use of the available horizontal space. With our current design there, the boundaries are visible if you look closely, and there is a way to get each timestamp. But I would not call that design a great success, as I suspect most users have never discovered it even if they've felt the desire for that information.

However:

The Figma has a design to support checking the date/time for any message; it looks like it wants us to let the user slide a message to the left, to reveal the timestamp: https://www.figma.com/file/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?type=design&node-id=147-8975&mode=design&t=P97AuMzjt47Mudyg-0 (I wonder if we can make the slide gesture reveal dates for all visible messages at once, like Messages on my iPhone, so you don't have to slide once for each message you want to check.)

We should implement that feature. And I agree that it'd be ideal for one swipe/slide gesture to reveal the times for all the messages in sight at once.