home: Add placeholder text for empty Inbox, Channels, and Direct messages

[chrisbobbe]

The "Direct messages" part of this was requested as part of #127 new-DM: it's item 0 in #1322 (comment) . This should block on the rest of #127, because the empty "Direct messages" placeholder says "Why not start the conversation?", and there's no way to do that pre-#127.

Screenshots coming soon.

cc @alya

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The Figma has some graphics on all of these, but we'll leave that for later: see issue #1551.

Fixes: #385
Fixes: #386

[chrisbobbe]

Inbox:

Before	After

[chrisbobbe]

Direct messages (when #127 is done, soon, there will be a "New DM" button at the bottom):

Before	After

[chrisbobbe]

Channels:

Before	After

[alya]

I'm pretty sure we had a CZO discussion about what to write as the second sentence in the empty inbox?

[chrisbobbe]

Ah yeah, that's right: #mobile > iOS feedback for Julia community member @ 💬

So e.g.

There are no unread messages in your inbox. You might be interested in Combined feed or Channels.

(but with links instead of italics).

Putting inline links (or italics, icons, etc.) in prose is blocked on getting it to work with internationalization (#1285), though. That version of a second sentence seems kind of useless without the links, so I think we should either leave it off or rewrite so that it calls attention to the nav buttons at the bottom. And file a follow-up issue for putting the links in there, noting that it's blocked on #1285.

[alya]

OK, we can do:

There are no unread messages in your inbox. Use the buttons below to view the combined feed or list of channels.

[chrisbobbe]

SGTM; updated (and updated the screenshots), and filed #1553 for the text with links.