Add "bite" activity support

[ibelieveinwhatileaveinthewaybehind]

Some fedi software (mainly Iceshrimp.NET, but also Iceshrimp-JS and Akkoma with patches) support the "bite" activity. It would be nice to have support for sending bites to users and notes in Moshidon.

Here are the routes that Iceshrimp.NET uses for these activities:

• POST /api/v1/users/bite (this is the only one used by the other implementations; IS.NET can use all 4)
• POST /api/v1/users/{id}/bite
• POST /api/v1/users/{id}/bite_back (biting back is a special kind of bite that only works if previously bitten by somebody)
• POST /api/v1/statuses/{id}/bite

I've figured out how to implement notification support for bites (and this wouldn't conflict with instances that don't support it), but I wasn't able to figure out implementing a way to send bites.