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According to prologic this is now the norm for yarnd. The parser already
handles them as well, so nothing to do other than changing the compose
form.

This refactoring also uncovered some kind of bug, that got masked by the
`twtxt.models.Tweet` constructor, which did the actual conversion to
seconds precision. Without that Tweet container in between anymore, two
precision-related tests failed. So this precision handling needed to be
replicated into the hash creation – where it acutally belongs. One day,
we're finally getting rid of the `twtxt.models.Tweet` entirely, so it's
not really duplicated anymore.

This is part of surgically removing the twtxt reference implementation
of tt.

Now this is probably more urwid-like with separation of functionality
and presentation. Although it is much uglier, I reckon. :-/