Reviewer recruitment: add a regex to validate comma separated values in 'Invitee Details'

[melisabok]

Users sometimes enter the recruitment list like:

'Reviewer name <email@address>'.

We should support either both or one of them. But we need to add an input validation so this error doesn't occur in the process function.

Reference: https://openreview.net/logs/process?id=B1eTx3w8UH