Send copy of call for speakers email to the speaker itself

[timiwahalahti]

When the speaker submits a talk to WordCamp, send a "receipt" of that to the speaker. It will help people keep track of what talks they’ve submitted to WordCamps, and when.

Instead of sending out separate emails and handling logic for that, I decided to just modify the email that goes out in any case. This helps to keep the code a tad cleaner, but has also the added benefit of organisers being able to reply to that email and continue the discussion with the speaker on a thread that has all of the submitted information. In addition, the same code can be extended for sponsor and volunteer applications easily.

Fixes #376

Props @melchoyce

Screenshots

How to test the changes in this Pull Request:

1. Head to WordCamp site and open a Call for Speakers post
2. Fill the form
3. Check the email (on Docker, use Mailcatcher for that)

[timiwahalahti]

The linting errors are due to action/filter list styling that has been the preferred way in WordCamp.org but for some reason, phpcs decided to nag about it...

[renintw]

Tested and saw the expected result.