Changes to camptix-invoices plugin

[timiwahalahti]

As the Community Team agreed to activate the Camptix Invoices plugin for all new WordCamps by default, I started looking at the plugin's codebase to identify if there are things that need some tweaks to make it work without organisers needing to update settings.

In this PR:

• Enable invoices by default
• Enable VAT field by default
• Use site date format as default in the invoice
• If all enabled payment methods use allowed predefined WPCS accounts, use WPCS address in the invoice. In other cases, default to the site name and ask organisers to update the setting.
• Makes address and VAT fields optional
• Prevents the email being sent if no attachment

Fixes #928, #255, #640
See #971

Props @javiercasares, @harmonyromo, @ciudadanoB, @dd32

Things identified that need work:

• Handle invoice sender details when WordCamps is not using WPCS to run money
• Possibly enable VAT field by default
• Activate invoices by default (the setting)
• Add plugin to list of network activated ones
• Make VAT field optional
• Network activate the plugin

Screenshots

Camptix invoices settings with new defaults.

Paypal payment method enabled with custom and Stripe enabled with WPCS, notice about company address.

All payment methods using WPCS, company address field hidden from invoice settings.

Invoice fields on tickets page.

Invoice PDF.

How to test the changes in this Pull Request:

1. Create new WordCamp site and activate Camptix Invoices
2. Navigate to Camptix settings, invoices tab -> invoices and VAT should be enabled
3. Go to payment tab, activate Paypal or Stripe with custom account -> you should see notice about company address
4. Activate the other payment method with WPCS account -> you should still see the notice
5. Change the first payment method to use WPCS account -> notice should dissapear
6. Go buy ticket, request invoice -> invoice should be sent and WPCS address should be in invoice