Change the 'user_exists' check when mapping upload_files cap

user_exists() is not a valid function name.

Fixes #320
Readme update
Disable upgrade notices for the time being

On some installations, the upgrade message never went away, which apparently
caused an infinite amount of annoyance for those users. Because pretty much
every user from the pre-1.2 Docs era has upgraded, I'm removing the auto-
upgrader for now, but leaving the code there if anyone needs it.
Readme
When a Doc is associated with a hidden group, do not display the affiliation to non-members

It remains possible to create a public Doc from a hidden group, but now non-
members cannot see the name of that group on the Doc or in public directories.

Fixes #316
Bumps tags to 1.4.1
Merge branch '1.4.x'