Added send test email option

[Schmidie64]

Pull Request for Issue joomla#22126 .

Summary of Changes

Added a send test-mail button for @Hackwar PR.
This option allows you to send a test-email with a specific mail-template chosen to every valid email-address.

Testing Instructions

1. Go to your Joomla adminsite
2. Change your url to http://{sitename}/index.php?option=com_mails&view=templates
3. In the left upper corner you can find the "Send Test E-Mail Button"
4. Now a modal windows will be open
5. Now you can enter the email-address where the email will be send to
6. Choose a template for the test mail
7. Press the "send test e-mail button"
8. Now you should received a email with the chosen template

[Hackwar]

How does that fill the data for the template?

[Schmidie64]

Currently it use hard coded values. This ist an Open question how to Deal with the dynamic value. Maybee @wilsonge can answer These question.

[wilsonge]

I honestly have no clue how to deal with template data :) You can do plugin events I guess but it kinda sucks because for components sending emails you then mandate plugins per component to send test emails. I also considered in the mail template table storing the extension and going through the bootComponent

[Schmidie64]

What do you think @Hackwar?

[Schmidie64]

For now i implemented a replacement for the placeholders in the test-email. You can change the placeholder-value via an language override. If there is a final decision on how to deal with this feature, then we can consult again about this function.

@roland-d