API Defaulting composer core dependencies to to @stable

Required to ensure correct operation of
"composer create-project silverstripe/installer my-path 2.4.10"
(checkout with tags).
Removed @stable markers from 2.4 dev branch
Better Brief skeleton additions
Changes to the installer to take account of our own skeleton changes
Including
- Removing tutorial theme - we add our own at run time
- Adding our logo to assets
- Adding our favicon
- Adding our Google Webmasters ID file
- Adding a robots.txt file
- Adding our skeleton to the code folder
- Adding our JS file for excluding admins
- Default config options
- Edits to Page.php
- Adding HomePage Page type
- .htaccess modifications to force www and other improvements
Adding default theme
Adding our default 2.4 theme
Also fixed a bug in .htaccess
Deleting install files

We don't use the installer via the web interface, so we have removed the
installer
Fixing scope and formatting

Fixing the scope of functions (declaring them)
Fixing formatting to the statics (for our own convention)
Minor tweaks

Fixing some minor formatting issues as well as efficiency improvements
Merge branch '2.4' into 3.0 & Adding 3.0 modifications
Modifying the 2.4 skel so it works with 3.0