Add support for IAB workshops as groups to the datatracker.

[rjsparks]

Look at adding IAB workshops as a new group type, allowing meetings and meeting materials.

cc @mirjak, @cindymorgan

[mirjak]

@rjsparks can we priories this? This is open since last summer and I thought we could be able to get this in rather quickly. Especially with the new webpage we now will rely on having all workshop information in the datatracker. Or what is the planned time line for this issue? I was hoping we get this done before the webpage goes live but we really need it before the next workshop in January.