Optimize the session data

Only store the first eight characters of any MD5 hash and never include
the relative path to the installation (`TL_PATH`), because it will be stripped
anyways in `System::getReferer()`.
Update the autoload and IDE compatibility files
Explicitly check whether `Model::getRelated()` returns a model
Fix the "removeSession" callbacks (see #5353)
Merge branch 'hotfix/3.0.6' into develop
Allow to create forward pages without a specific target (see #5453)
Add the cookie path to the debug console (see #5460)
Show invisible elements in the back end preview (see #5449)
Load jQuery before MooTools (see #4890)
Show the submit button label in the `FormSubmit` widget (see #5434)
Explicitly check whether `Model::getRelated()` returns a model
Remove the ellipsis character from error messages and exceptions (see #4971)
Revert "Load jQuery before MooTools (see #4890)"

This reverts commit 323e77b263ab318d3e77150c6cf69da32ac230eb.
Load jQuery before MooTools (see #4890)
Add the "allowTransparency" attribute to the mediabox script (see #5077)
Support decimal commas in widgets and convert them automatically (see #3488)
Hide the up link in the book navigation if it points to a forward page (see #5374)
Fix the "hash synchronization" message (see #5447)
Update all language files
Add "ogg" and "oga" to the `.htaccess.default` file (see #5455)