Add /avatar set command to publish avatar

Use `/avatar set <path>` where <path> is an image file to upload a new
avatar for the current user. When the avatar is too big it gets scaled
down. Scaling code copied from dino.

Fixes https://github.com/profanity-im/profanity/issues/1687

tests/unittests/xmpp/stub_avatar.c

@@ -8,3 +8,8 @@ avatar_get_by_nick(const char* nick)
 {
     return TRUE;
 }
+gboolean
+avatar_set(const char* path)
+{
+    return FALSE;
+}