Homepage UI for editing metadata file

[simonw]

Since we are going to have a metadata file which sets the title/description/etc for each database, why not allow you to run the app in —dev mode which makes the homepage into a WYSIWYG editor that can save to that file format.

[simonw]

I ditched the metadata file concept.

[simonw]

Re-opening this - I've decided to bring back this concept, see #68

[simonw]

See also #138

[simonw]

I think the datasette skeleton command from #164 makes this obsolete.