Mattermost-server: Render custom emoji in custom branding text

[mattermod]

1. Create a custom emoji for "yay"
2. Go to System Console > Customization, enable custom branding and enter the following as the custom branding text

:yay:

Observed: Custom emoji doesn't render

Expected: Custom emoji renders.

NOTE: Also applies for terms of service feature.

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JIRA: https://mattermost.atlassian.net/browse/MM-12657

[cinlloc]

:hand: I would like to work on this one.

[cinlloc]

I found that entities.emojis.customEmoji map is empty in login screen state. It appears that markdown component needs this map to render custom emojis properly. I tried to initiate the map using emoji.ts#getCustomEmojis in root.jsx but unfortunately it doesn't work (authentication error in promise). Do you think I'm on the good way or should I change the approach?

[cinlloc]

Seems there is an internal debate about security issues with this bug (cf. https://mattermost.atlassian.net/browse/MM-12657). Could you kindly double-check that the feature is wanted?