Resolve the emoji.json "unicodes" mess

We now operate on a new set of assumptions:

- Characters with VARIATION_SELECTOR_15 shouldn't render as emoji, even if OS X doesn't respect that currently. This removes explicit aliases that include VARIATION_SELECTOR_15.

- VARIATION_SELECTOR_16 is optional for most characters to render as emoji on OS X. For those that *don't* have it optional, we include VARIATION_SELECTOR_16 in their raw representation in `emoji.json`. Other characters list their form including VARIATION_SELECTOR_16 implicitly in `unicode_aliases`.

- For emoji that consist of 2 characters + variation selector, we assume that the selector can come between the 2 characters or *after* them, so `find_by_unicode` now supports both forms.

- The `db/aliases.html` script ensures that `emoji.json` only contains characters that Safari on OS X actually renders as emojis.

This eliminates the need to have `unicodes` key in emoji.json, which simplifies things a bit.

Reverts parts of #56 but continues in the same philosophy: only render emojis that OS X renders by default.

/cc @aroben