Emoji, discussion composer transforming codes incorrectly

## Explanation

(Probably the 100th issue created about this specific feature lol)
Currently, in any installation of Flarum and even discuss.flarum.org, Emoji's behave in a weird manner when doing the following:

* Choose a thread to post in.
* Start a reply to bring up the composer.
* Created a list of 2-3 random Emoji short-codes that work.
* Go to the 2nd line and backspace until your cursor is right next to the end colon of the send emoji.
* Press enter for a new line and watch as the Emoji changes to either 💯 or something you didn't even pick.

Two examples have been displayed in the "Screenshots" section below.

## Technical details

- Version of Flarum: Beta 7.1 and Master
- Website URL where the bug is visible: This can be reproduced in any discussion really.
- Affected Browsers: Using Firefox Developer Edition (Latest)

## Screenshots

First one is copied and pasted short-codes and pressing the "Enter" key right after each colon:

The second one was done by just typing out the short-code you want and selecting from the picker and then pressing "Enter" after backspacing after the colon:

Please feel free to edit the title to something less confusing and appropriate.

Reproduce the issue in the discussion composer using the listed steps: enter several working emoji short-codes, move to the second line, backspace to the ending colon, and press Enter. Compare typed or picker-selected codes with the screenshots; done means pressing Enter no longer transforms the chosen short-code into a different emoji.

Written by the indexing model from the issue text.