Redesigned DM recipient headers

[sirpengi]

This is built on top of #416.

This PR updates the DM recipient headers to the new style. Text style and spacing was taken from the same sources as #416:

• https://www.figma.com/file/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=538%3A20849&mode=dev
• UI redesign: message content layout zulip-mobile#5511

But given there were no mockups that included a DM recipient header the style used here also looked to how web renders these headers (in particular the color to use for the background, and icon to use for display).

Here showing behavior when the system requests a larger font:

[sirpengi]

@gnprice rebased on main and ready for review!

[gnprice]

Thanks @sirpengi! Generally this looks good — a few small comments.

[gnprice]

msglist: Remove HitTestBehavior.translucent on DmRecipientHeader

This seems unintended as the header should capture all
clicks within its bounds.

Capturing all clicks within its bounds is the reason this was translucent. Here's that value's doc:

  /// Translucent targets both receive events within their bounds and permit
  /// targets visually behind them to also receive events.
  translucent,

So with that value, the header does capture all taps within its bounds.

The default is deferToChild. That one's doc says:

  /// Targets that defer to their children receive events within their bounds
  /// only if one of their children is hit by the hit test.
  deferToChild,

So with that default, the header would not necessarily capture all taps within its bounds — it only does so if its child is hit.

This parameter is unnecessary now, though, so it's good to remove it. The reason it's unnecessary is that the child fills the whole box.

Looking at the history with git log -L (git log -L 668,693:lib/widgets/message_list.dart as of the tip of your branch, to cover this whole return statement), I believe the key change was 412233c. Before that, and when this line was added in f7feabf as part of making the DM recipient header touchable at all, the child was an Align and so it only occupied the left-hand portion of the parent. That made HitTestBehavior.translucent necessary in order for touching everywhere in the header to work.

[gnprice]

Suggested change

	"description": "Message list sticky header for a dm group with others.",
	"description": "Message list recipient header for a DM group with others.",

"DM" is an initialism, so in normal English text (as opposed to identifiers in code) it's all caps.

Also although these headers are sticky, I think "recipient header" works better as a name for them because it points more to what they're about.

[gnprice]

Suggested change

	"messageListGroupUnknownUser": "(unknown user)",
	"unknownUserName": "(unknown user)",

I think it makes sense to use the same form for this pretty consistently across the app — seems helpful for reducing confusion about whether it literally means an unknown user or the name some actual user has picked. So we can make the identifier more general.

[gnprice]

Suggested change

	"description": "Name placeholder to use for user when we don't know their name."
	"description": "Name placeholder to use for a user when we don't know their name."

The headline-ese "user" without an article sounds like it might mean "the user", i.e. the self-user.

[sirpengi]

@gnprice ready for review again!

[gnprice]

Thanks for the revision! All looks good — merging.