Add email tag for admins to be informed when a donation was matched

[mathetos]

User Story

As a GiveWP admin, I want to get emails of my donations and have an indication of whether the donation was company matched or not as well as what company was indicated for matching purposes.

Acceptance Criteria

• An email tag called {company_matching} is created
• The {company_matching} tag outputs "None" if no match was chosen; and the company name if a match was chosen. It additional provides a link to the company matching page for easy reference.