Option for default presentations on tenants

[paul1278]

Description

This pull request adds the option to supply default presentations on the create-call on a per-tenant basis. This is a good addition to the default params / override options.

When submitting a POST-request as create-call, the supplied body will be used. If there is no body supplied or it is initially a GET-request, the default presentations for the current tenant will be added.

This PR is independent from #965. I suggest merging the other one first.

Testing Steps

1. Add a tenant
2. Add default presentations for this tenant:

./bin/rake tenants:presentations[<id>,"https://test.bigbluebutton.org/default.pdf->default_1.pdf;https://test.bigbluebutton.org/default.pdf->default_2.pdf"]

3. Create a meeting and see the default presentations.

[pielonet]

Hi,

Thanks for this PR that we have been waiting a long time for.
Please release a beta version we can test soon.