Increase check time for emote sets

Ref #3
`window.CurrentChat.user_to_emote_sets` doesn't populate right away--it
is delayed. The previous fix was to just refresh until emotes appeared,
but this only worked because `user_to_emote_sets` actually populated
quickly enough. Upped the check time to 20 seconds, but may be adjusted
in the future.
Change where emotes usability is checked

Changed to checking after we know for sure what the emote sets are.
Previously, it was checking right when the API returned the emotes, but
by then the emote sets weren't known. This will go hand-in-hand with
a05b2ba85895409e3db45a1f0ae28c9e668d3224
Fix emote usability check. Ref #3

Previously, checking usability when API call was done. As emote sets
weren't always populated at the time the API finished, sub/turbo emotes
weren't always shown.

API call will now push to `rawEmotes` and `emotes` will be the usable
ones. `checkEmotesUsable` will run every time `populateEmotes` is run.
Although it may take some time to get the proper emotes, it is now in
line with the way Twitch handles it.
Fix sub emotes color & remove let keyword

Subscription emotes don't get their color the same way they used to.
Forget to include this change while doing the other commits in #3

Also changed "let" back to "var" as Firefox is still the only browser to
support it.