Add credits functionality and update database schema

[o-psi]

This pull request adds the ability to add credits to a client's account. It includes changes to the database schema to support the new functionality, as well as new PHP files for handling credit-related requests. The changes also include updates to the client payment page to display credits and allow for applying them to invoices.

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[wrongecho]

Test these changes at: https://bulkpayment808.pr-review.itflow.org
(automatic message)

[johnnyq]

This looks exciting, cant wait to see the finished product. DB Scheme looks good too

[johnnyq]

for reference https://github.com/itflow-org/itflow/issues/193

[sonarcloud[bot]]

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