Feature/invoice sending

[Domin1kG]

Description

This PR includes changes, which make it possible to send invoices to clients, the possibility to use email variables inside of a message, allows multiple address to be entered in the CC/BCC fields, also added automatic emails for when an invoices enters the 'Pending' state, when it becomes 'Paid' or becomes 'Overdue'.
Created a new modal file, based on the one for bulk emails and added more email templates for the various invoice states.

Checklist

• Ran the Black Formatter and
  djLint-er on any new code (checks will fail without)
• Made any changes or additions to the documentation where required
• Changes generate no new warnings/errors
• New and existing unit tests pass locally with my
  changes

What type of PR is this?

• ✨ Feature

Added/updated tests?

• 🙅 no, because they aren't needed

Related PRs, Issues etc

• Related Issue idea: Allow invoice sending #454
• Closes idea: Allow invoice sending #454

[TreyWW]

Hi @Domin1kG apologies for the delay, I will try and look at this soon.