Add link to zip download

Adding the complementary part to mkly's changes, making it more friendly for newcomers to know how to get the latest development build without having to know how to navigate GitHub.
Remove hardocded textarea width from form block output
Added file description to data returned by the files/get_data tool
Bugfix: External link bug

If you install concrete5 to document root and put page list block under
2nd level then the external link doesn't work.
set fixed height in slideshow block
determine max height of slideshow images
constant to hide concrete5 version number

as mentioned here http://www.concrete5.org/community/forums/chat/visible-version-number-in-source-code/
don't output unnecessary jquery UI in form block

Take 2 (see https://github.com/concrete5/concrete5/pull/672 for prior discussion).
Also outputs CSS to header (not footer) if needed.
Fix blocktype sort mismatches

Removes gaps in the btDisplayOrder sequence whenever a blocktype is deleted. This prevents future problems when sorting the remaining blocktypes in the dashboard list.
Fix: Login loop with cookies disabled

When trying to login with cookies disabled no error message is shown to
the user. Some 'corporate' networks have this setup by default, I have
experienced it recently with Concrete & a client.

The patch below modifies the login controller to throw an exception on
cookies being disabled.
Close two not closed <div>
Remove extra </div>
Remove extra </div>
Remove extra </div>
Move misplaced start of form
Fix start of form and closing divs
Remove extra </div>
Remove extra </div>
Add missing </div>
Add missing </strong> and removed extra </form>