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

src/command/cmd_defs.c

@@ -2443,16 +2443,20 @@ static struct cmd_t command_defs[] = {
       CMD_TAGS(
               CMD_TAG_CHAT)
       CMD_SYN(
+              "/avatar set <path>",
               "/avatar get <barejid>",
               "/avatar open <barejid>")
       CMD_DESC(
+              "Upload avatar for oneself (XEP-0084). "
               "Download avatar (XEP-0084) for a certain contact. "
               "If nothing happens after using this command the user either doesn't have an avatar set at all "
               "or doesn't use XEP-0084 to publish it.")
       CMD_ARGS(
+              { "set <path>", "Set avatar to the img at <path>." },
               { "get <barejid>", "Download the avatar. barejid is the JID to download avatar from." },
               { "open <barejid>", "Download avatar and open it with command." })
       CMD_EXAMPLES(
+              "/avatar set ~/images/avatar.png",
               "/avatar get thor@valhalla.edda",
               "/avatar open freyja@vanaheimr.edda") },