move emoji/15.0 files to emoji/dev

[markusicu]

... as agreed for all unreleased files, and consistent with the ucd/idna/security/... files.
The emoji files for 15 had been misplaced in PR #232.

Also sync the two data files in the repo that are still duplicated between data/emoji and data/ucd. (See issue #152.)

Also update the two emoji readmes, and
remove the emoji/dev/ReadMe-ucd-emoji.txt which was a copy of ucd/dev/emoji/ReadMe.txt.

[nedley]

Please review https://github.com/unicode-org/emoji/pull/24 and take that version of emojiOrdering.txt.

[markusicu]

Please review unicode-org/emoji#24 and take that version of emojiOrdering.txt.

For this PR here, I would prefer to just move the existing data files into the right place, rather than also change the data, especially from a pending PR.

Also, I don't see any "ordering" file in the other PR with this name or contents.

I can rubber-stamp https://github.com/unicode-org/emoji/pull/24, if you like, but not much more; and it still has an unresolved negative review from yourself.

[nedley]

For this PR here, I would prefer to just move the existing data files into the right place, rather than also change the data, especially from a pending PR.

Also, I don't see any "ordering" file in the other PR with this name or contents.

Sigh, there are too many unreviewed PRs in flight… Just do what you have to do.