Create Migration for channel members to run participants

[crspeller]

There must be a migration from members of a channel to the new run participants table. This migration must only run once. Anyone who is a member of the channel associated with the run channelid must become a participant. The list of channel members and run participants should be identical at the end of the migration. This should not change the permission level they have within the run.

This may be able to be done with a single INSERT INTO IR_Run_Participants SELECT DISTINCT. See https://github.com/mattermost/mattermost-plugin-playbooks/blob/0cd966b186b0c1d805620f30d4cc8e244f8fdd95/server/sqlstore/migrations.go#L2110 for a similar query.