toaster: Show mode-appropriate landing page

toaster: Show mode-appropriate landing page

The same landing page is shown for both analysis and
build modes. This means that users in analysis mode can see
options which are not available or broken in that mode.

Modify the landing page template to show a simple "run a build"
message if the user is in analysis mode and has no builds yet.

Also clean up the landing page HTML, because the indentation was
a mess and the HTML was invalid.

[YOCTO #8514]

Signed-off-by: Elliot Smith <elliot.smith@intel.com>
Signed-off-by: brian avery <avery.brian@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>