Enforces a new first-run setup wizard for access control to be run and forbids running OctoPrint as root unless a special command option is supplied

The dialog also informs about the risk of unauthorized strangers (mis)using the printer if an unsecured OctoPrint installation is made available on the internet.
Only enable "Keep Enabled" button if data entered into form is valid. Also changed the wording a bit here and there.
And a robots.txt for good measure...
Properly enable serial logging on startup if configured
(cherry picked from commit 2261ddf)
Only match "rs"/"resend" on the start of a line, otherwise the wrong lines might be interpreted as resend requests
(cherry picked from commit ae09a50)
Check if self._comm is None before trying to select a file...
(cherry picked from commit 5631a45)
Added .gco to supported extensions
(cherry picked from commit d51aa65)
Properly handle failed uploads: display message, reset upload status
(cherry picked from commit ce7ccc1)
Merge pull request #1 from foosel/master

rebase