Add "/avatar disable" to comply with point "3.5 Publisher Disables
Avatar Publishing" in XEP-0084.
src/command/cmd_defs.c:2416
Add "disable" argument. Reword the "/avatar" command description
so it flows better.
src/command/cmd_ac.c:1101
Add "disable" to the "/avatar" autocomplete dictionary.
src/command/cmd_funcs.c:9277
Split "/avatar" commands into two groups with an if statement; those
with a parameter and those without. "cons_bad_cmd_usage()" is in both
groups, which is messy. "disable" has similar logic to "set", but it
includes a failure message.
src/xmpp/avatar.c:152
"avatar_publishing_disable()" uses the same logic to publish metadata
as in "avatar_set()".
src/xmpp/avatar.c:238
Add a message to inform users when they do not receive an avatar after
using "/avatar get" and "/avatar open". In case of a failure, the user
will be subscribed to future avatar updates as long as they continue
to use their current instance of profanity. Adding
"caps_remove_feature()" after "cons_show()" will unsubscribe the user
from avatar updates and prevent the user from downloading an avatar
unexpectedly hours later without issuing an "/avatar" command.
src/xmpp/stanza.c:2698
The new "disable" function follows the same logic as
"stanza_create_avatar_metadata_publish_iq()".
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
