Custom emoji alt attribute should include the colons

...so that you can copy paste them and they still work.

It is like that in mastofe, ftr.

mastofe

<img draggable="false" class="emojione custom-emoji status-emoji" alt=":yell_right:" title=":yell_right:" src="https://piggo.space/emoji/custom/yell_right.png" data-original="https://piggo.space/emoji/custom/yell_right.png" data-static="https://piggo.space/emoji/custom/yell_right.png">

pleromafe

<img src="https://piggo.space/emoji/custom/yell_right.png" alt="yell_right" title="yell_right" class="emoji">

I'm not advocating for all the mastodon bloat, but this is good