Invoices: Add invoice and reminder emails to platform

[mariobehling]

Please add the invoice email templates here to the system and send them automatically on the desired day to the "owner" of an event as follows:

• Invoice Email on 1st of each month (as defined in https://eventyay.com/admin/settings/billing)
• Invoice Reminder Email after 14 days
• Invoice Reminder Email after 27 days
• Invoice Past Due Date after 30 days

For admins:

• In BCC send the email to the admin email of the platform
• Ensure they are visible at: https://eventyay.com/admin/messages/list

[iamareebjamal]

Did not add these in eventyay.com/admin/messages/list as:

• The page/setting does not work. We do not use the subject/bodies from here
• Will need a migration strategy, thus this must be handled in notifications milestone
• BCC implementation is still left because there is no admin email setting and previously there was no BCC functionality in the platform, so that'll take some extra implementation

[iamareebjamal]

Last point left so reopening

[iamareebjamal]

2 points are left #5147 (comment)

However, they are a bit complicated. You can work on them if you want to but they are not beginner-friendly

[sb39]

@iamareebjamal I want to take this up if available