#3671262 Recurring payments: Limit welcome email content to 2000 characters

Changes proposed in this Pull Request

Limits the max length of the Recurring payments’ custom confirmation email messages to 2000 characters.

The “Save” button is disabled if the entered message exceeds the max length:

Note that “Save” button is now a primary button as per @sixhours‘s suggestion.

Testing instructions

• Set up Recurring Payments on a site using the testing store. More info in PCYsg-lW4-p2 #sandbox-method.
• Go to Tools > Earn > Collect recurring payments > Recurring Payments plans.
• Add or edit an existing recurring payment plan.
• Click on the Email tab.
• Make sure the “Custom confirmation message” field is limited to 2000 characters.
• Make sure the plan cannot be saved when the field exceeds the max length.

Part of #42536