[DONE] Feature: Webinar management in the meetings module

[LoicBonavent]

Add webinar management in the meetings module:

• Add webinar parameters: USE_MEETING_WEBINAR, MEETING_WEBINAR_SIPMEDIAGW_URL, MEETING_WEBINAR_SIPMEDIAGW_TOKEN, MEETING_WEBINAR_FIELDS, MEETING_WEBINAR_AFFILIATION, MEETING_WEBINAR_GROUP_ADMIN
• Manage webinars fields (is_webinar, show_chat, enable_chat), classes (Livestream and Ingester) and rules
• Manage SIPMediaGW API to start / stop / toggle and send a message to this RTMP Gateway
• Add actions buttons to manage a webinar (restart a live, stop a live, stop webinar (meeting and live))
• Add REST meeting views