Add UI for managing shared files

We have no user interface for managing which files are shared. Add a tab
to the user interface where the user can do that.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Respond with content of requested file

When responding to requests we send a debug response, which is just a copy
of the request that the user agent sent. While that is useful we really
want to send the file that the user agent requested.

Look up the requested file and send it back to the user agent if it is
in the list of shared files.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Don't allow adding file's with duplicate file name

We have not implemented support for subdirectories, which means that we
can't share multiple files with the same file name. We don't check for
this when new files are added. If the file name is requested then the
first one found will be returned.

Check for duplicate file names and prevent them from being added when
adding new files.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>