Deactivated custom emoji offered in autocomplete

[gnprice]

A custom emoji can be deactivated:

• https://zulip.com/help/custom-emoji#deactivate-custom-emoji

When that happens, it's still valid for existing messages and reactions; but the server will reject new uses, and so it shouldn't be offered in the UI.

Currently we don't take account of that, so we offer deactivated custom emoji the same way as those still active.

The effect is conspicuous on chat.zulip.org, because there are a substantial number of deactivated emoji (57 of them):

$ <tmp/register.json jq '.realm_emoji[] | .deactivated' | sort | uniq -c
    131 false
     57 true

Particularly so after #1112 (implementing #1068), since custom emoji are ranked ahead of Unicode emoji (other than the "popular" emoji).