If we migrate gameDuration into a customGame field

[jimmydorry]

Raised here: https://github.com/GetDotaStats/stat-collection/issues/55

We need to:

• [ ] Update the s2_custom API to update the gameDuration field under s2_match table if gameDuration field is found in the schema.
• [ ] Update the schema admin form to pre-fill gameDuration value
• [ ] Update all of the schemas to add this gameDuration field