Pleroma users with a custom emoji in their display name can not follow channels

An error is thrown if a user with an emoji in their display name attempts to follow a Funkwhale channel.

Feb 25 12:24:05 pr-funkwhale gunicorn[17063]: 2021-02-25 18:24:05,659 funkwhale_api.federation.authentication INFO     Discarding HTTP request from actor/domain https://pirateradio.social/users/captain, {'tags': {0: {'type': [ErrorDetail(string='"http://joinmastodon.org/ns#Emoji" is not a valid choice.', code='invalid_choice')]}}}

The tags includes a url to the emoji(s) used in the display name and the serializer won't validate it.