Update chat/ejabberd to 16.12.

API / integration
- New API permissions framework

Commands
- Add configurable weight for ejabberd commands
- add_rosteritem: Support several groups separated by ;
- create_rooms_file: Fix reading room jids from file
- delete_old_messages: Fix command for SQL backends
- send_message: Don't duplicate the message
- Remove obsolete remove_node command (use leave_cluster)
- Fix reload_config
- Cleanup mod_admin_extra, add few functions
- Expose unregister API command

Core XMPP
- New BOSH module
- Use fxml_gen XML generator
- Use our new stand-alone XMPP library instead of jlib.erl
- Don't let MAM messages go into offline storage
- Add xdata generator for XMPP data form
- Get rid of excessive (io)list_to_binary/1 calls

HTTP
- Add authentication support to mod_http_fileserver
- ejabberd_http: Handle missing POST data gracefully
- Use inets instead of lhttpc in http_p1
- Add http_p1.erl, rest.erl, and oauth2 ReST backend for OAuth2 tokens

MUC
- Create room on configuration request as per XEP-0045, 10.1.3
- Ensure that presence_broadcast room option is stored
- Fix conference disco#items when running multiple virtual hosts
- Fix Result Set Management (RSM) for conference disco#items
- Introduce muc_invite hook
- Make the constant MAX_ROOMS_DISCOITEMS configurable
- mod_carboncopy: Don't copy MUC private messages

MUC/Sub
- Store the flag "Allow Subscription" room option in database
- When getting list of subscribed rooms, also check temporary ones
- Add password support in muc_subscribe
- When unsubscribes, check if room should get closed

Pubsub
- Enforce pubsub node removal
- Relational databases support
- Append ; to privacy_list_data exporting lines
- Improve relational database import

Build
- Make build system compatible with rebar3
- Produce ejabberd.service and fix for systemd usage
- Cleanup ext_mod and fix compilation path
- Fix compilation of external module with new xmpp lib

(fhajny)