inbox: Change empty-inbox text so it links to "Combined feed" and "Channels"

[chrisbobbe]

This issue is blocked on #1285.

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With #1552, we'll have some plain text as a placeholder when the inbox is empty:

We should change it to say something like:

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

except instead of italics, we put links to those pages. (And/or to "Recent conversations", which isn't implemented yet; that's #315.)

Putting inline links in prose is blocked on getting it to work with internationalization; that's #1285.

Related

• home: Add leading/trailing graphics to empty-page placeholders #1551