feat(model): Changed the UserReport to have a flexible foreignkey to environment.

[lauryndbrown]

This is part of the Sentry 9 environments change. We want to filter UserReports by environment, but there currently doesn't exists an easy way of doing so. After talking with Ted and Matt, we decided to add an additional environment column to UserReports model to facilitate this filtering.

[ghost]

	2 Warnings
⚠️	You should update CHANGES due to the size of this PR
⚠️	PR includes migrations

Migration Checklist

• new columns need to be nullable (unless table is new)
• migration with any new index needs to be done concurrently
• data migrations should not be done inside a transaction
• before merging, check to make sure there aren't conflicting migration ids

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