Allowing for subscription transaction IDs up to 64 characters

[dparker1005]

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Changes proposed in this Pull Request:

Some gateways, such as Square, have subscription transaction IDs that are longer than 32 characters. This PR bumps our limit to 64 characters which matches the limit that we already have on payment transaction IDs.

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[ideadude]

Good to merge. Shouldn't be any issues with this DB delta.