when you create a mutable file in the WUI you should get a nice user interface page back

[zooko]

Currently when you create a new mutable file with the "Upload a file" form on the welcome page of the WUI, you get back a text/html-type page with a mutable file cap in it and nothing else. Instead you could get back a page that confirms what you've just done and gives you controls for things you could do with your new mutable file. It might also want to remind you that if you lose all copies of the cap then you've permanently lost the ability to access this file.

This is probably a sub-task of #277. Someone who wants to start on #277 might like to start with this one as a first job.