Update download_layout to serve layout
Update download_layout to serve layout

This commit changes `download_layout` view to dump the created
layout to a file and then serve it using
`send_from_directory`.

Caveat: Currently each layout is dumped `root.layout` and therefor
overrides a previously created layout.

in-toto/layout-web-tool,in-toto/layout-web-tool,in-toto/layout-web-tool