Rename custom_record to custom_bbb_record and override welcome message when appropriate

I'm re-applying this code after reviewing the 0.1.1 version of bbb-lti. From my testing, an LTI's custom welcome message is currently being ignored.
Merge pull request #107 from kishba/fix-lti-custom-welcome-message

Fixes LTI custom welcome message for bbb-lti 0.1.1