Adding community to webcast requests

[jbenito]

This will allow the webcast team to create public/private webcasts. The idea is:

• Add to the webcast form a drop-down menu with choices (e.g. ATLAS, CMS, public). Public should be the choice by default.
• Those choices are configurable from the plugin admin interface, so we allow to specify the different types.
• The types should have a title and a "webcast" url (in the plugin admin interface) in order to be able to generate the different webcast links.
• When the webcast manager accepts the webcast, the webcast URL link for that event will be created depending on the chosen type.