install: move table spacing to CSS

validate.nu:
    Error: The “cellpadding” attribute on the “table” element is obsolete.
    Use CSS instead.

http://stackoverflow.com/questions/339923/how-to-set-cellpadding-and-cellspacing-in-css
install: fix breadcrumb links to use site_url()

i.e. they wouldn't work for subdirectory installs
install: fix new links back to main Bonfire

i.e. for subdirectory installs.

I used base_url() instead of site_url() to avoid adding the /index.php/
segment that the installer uses.
install: fix "already installed" lang() key