Add BookWyrm user-agent to http requests

This allows other software to identify BookWyrm in calls, as well as
will allow BookWyrm to differentiate between calls done from other
fediverse software and BookWyrm to answer with specific BookWyrm data.

bookwyrm/broadcast.py

@@ -3,7 +3,7 @@ import json
 from django.utils.http import http_date
 import requests
 
-from bookwyrm import models
+from bookwyrm import models, settings
 from bookwyrm.activitypub import ActivityEncoder
 from bookwyrm.tasks import app
 from bookwyrm.signatures import make_signature, make_digest
@@ -79,6 +79,7 @@ def sign_and_send(sender, data, destination):
             'Digest': digest,
             'Signature': make_signature(sender, destination, now, digest),
             'Content-Type': 'application/activity+json; charset=utf-8',
+            'User-Agent': settings.USER_AGENT,
         },
     )
     if not response.ok: