Add affiliate commission rate option

[michaelbeil]

Is your feature request related to a problem? Please describe.
It would be nice to set a commission rate before an affiliate is created.

Describe the solution you'd like
Allow site owners to have an option to set the commission rate.

This came up in ticket 475898.

[andrewlimaza]

For context here, the feature spoken about is to allow developers to set a predefined commission rate when creating a new affiliate.

This would only work for new affiliate creations and not adjust everyone's rate (pre-existing affiliates etc).

The idea would be to make the default values that are loaded when creating a new affiliate to be filterable via code -> https://github.com/strangerstudios/pmpro-affiliates/blob/dev/adminpages/affiliates.php#L104 (around here)

[michaelbeil]

A customer (ticket 479981) would like this:

I do not have the option to set commission percentages when creating an affiliate.

[dwanjuki]

A customer (Ticket 498913) is interested in this functionality.

I want to configure a comission rate for the affiliates that subscribe to a plan. For Example: Plan 1: 20% ; Plan 2: 30%; Plan 3: 40%. Is that possible?

[michaelbeil]

A customer (ticket 510104) would like commission to be automatic:

I want to give to all affiliates 50% by default. Can it be automatic for all the new subscribers that become affiliate automatic.

[MaryOJob]

Another request to set default: #586578