[imp][#30151] multiple recipients per contact form. Closes #734

## This is an updated PR of #734
Add ability to add recipients for all emails sent through a com_contact form. A field is added to the parameters of the contact form for listing one or more email addresses to be added to the recipient list.

At times organizations want multiple email addresses to receive a copy of emails coming through a contact form. This could be a temporary assignment (such as when the contact person is on vacation), or it could be a on- or off-site account providing a backup or a log of all inquiries.

This feature adds the additional recipients through the form instead of through the contact's email field. Currently the field validation for com_contact does not allow for multiple email addresses, so this new feature respects that expectation (and a potential dependency).

The submitted implementation requests a comma to separate emails, but it also handles a semicolon as a valid separator. An invalidly-formed email address will not be included into the recipient list, but all preceding email addresses are successfully included. The implementation trims white spaces.

## Test instructions

Create test users and a menu item type that will generate a form for at least one contact. The Contact menu item types are: Single, Featured, Categories, Category. Tip: When testing requires a validly-formed email address, the email address does not have to actually reach a live person for this feature to work. As long as you can receive one of the email addresses, you will be able to see what email addresses have been included on the To and CC lines. From Contact Manager->Options-> Form, add a valid email address under Send Copy To. All emails sent through any com_contact form should be including this email address on the CC line. This field accepts multiple email addresses as long as they are delimited by a comma. Also from Contact Manager->Options-> Form, add a valid email address into Additional Recipients. Also test for multiple email addresses using a comma delimiter. Test by sending an email using a Contact form. You should see the email addresses you have entered: the Additional Recipients will show on the To line, and the Send Copy To on the CC line. For each of the four Contact menu item types, go to AdvancedOptions->MailOptions and test values in Additional Recipients. If any value is added here, it will override the Additional Recipients set in Contact Manager. Note that Featured, Categories, and Category menu items generate lists, and they provide the settings for the contact form only if the contact is not associated with a more specific menu item type. So to test the settings under the Featured menu item, the contact to test must not be associated with a Single Contact form nor with a Category Contact form. I found the easiest way to test these list-based menu items was to unpublish all other Contact menu items during that test. Finally, edit a contact and from Contact Form experiment with Additional Recipients. Any value entered here will override any additional recipients set on either the menu item or through the Contact Manager. In all tests of Additional Recipients, the email address(es) set in SendCopyTo should always appear in the CC line.
Missing $params = $app->getParams();