Improved the code that generates the ssh keys for clients and shell users and reorganized it into a central function.

- fixed some smaller things in plugins for apache and nginx

Implemented #4271 Creating vHost Website Alias copies default index.html thus breaking the original site if it's based on index.php (or something not index.html)

- Added missing shell escaping

How to reproduce issue

Update bind_plugin.inc.php to correct "PHP Parse error:  syntax error, unexpected ':' in /usr/local/ispconfig/server/plugins-available/bind_plugin.inc.php on line 279"

Wrong table used to remove deleted backup record for website. File gets removed but not database record.

Do not add www as subdomain to a letsencrypt ssl cert when the domain of the webiste is already www.example.com (and not example.com) even if auto subdomain is set to www.

This reverts commit 7827db36