Make gpEasy sites portable by using $dirPrefix in content urls

Currently moving a gpEasy installation across directory levels is virtually impossible without in-depth knowledge of the system.

If gpEasy would save paths of images and internal links with $dirPrefix, a website could be truly portable and therefore copied/moved inside the file system hierarchy or from a test host/sub-dir to a live host/root-dir and vice versa.

current file sections array:

$file_sections = array (
  0 =>
  array (
    'type' => 'text',
    'content' => '<a href="/gpEasyCMS_test/About">Internal Link</a>
      <img src="/gpEasyCMS_test/data/_uploaded/image/someImage.jpg" style="width:100px; height:100px;" />',
    'attributes' => array (),
    'resized_imgs' => array (),
    'modified' => 123456789,
    'modified_by' => 'user',
  )
);

prefixed:

$file_sections = array (
  0 =>
  array (
    'type' => 'text',
    'content' => '<a href="' . $dirPrefix . '/About">Internal Link</a>
      <img src="' . $dirPrefix . '/data/_uploaded/image/someImage.jpg" style="width:100px; height:100px;" />',
    'attributes' => array (),
    'resized_imgs' => array (),
    'modified' => 123456789,
    'modified_by' => 'user',
  )
);

If implemented into gpEasy's core save functions, it would also work for addon data as long as plugins utilize these save functions.

Additionally the Rewrite Rules in .htaccess would have to be checked to accomplish this and maybe some other locations, too(?).

Shouldn't notably impact rendering performance.