API credentials validation does not clean up hostname

[kaittodesk]

Bug description
When using FQDN value in subdomain field during API credentials validation, the validation fails.

Steps for reproducing
Steps to reproduce the behavior:

1. Go to edit a Contact Form;
2. Click on Smaily for Contact Form 7 tab;
3. Fill out API credential fields, where subdomain field contains a fully-qualified domain name (e.g. example.sendsmaily.net);
4. Click on Verify credentials button;

Expected behavior
API credentials are verified and select with autoresponders is shown.

Actual behavior
API credentials verification fails.

Versions
Wordpress: 5.4.1
Contact Form 7: 5.1.7
Smaily for Contact Form 7: 1.0.3

Additional context
I think the issue is that subdomain clean up regular expression is broken. There's a lot of whitespace and array(...) in the expression:

https://github.com/sendsmaily/smaily-cf7-plugin/blob/master/admin/class-smaily-for-cf7-admin.php#L335-L349