API Removed non-functional $join argument from DataObject::get() and other APIs

See 68bb74820
Correct dimensions for preview overlay

During the side-by-side refactoring, those were removed,
rendering the overlay invisible, which lead to the "blocked"
state not being shown at all.
Only show preview switch if states are available

We've removed all states from RedirectorPage in order to disable the redirect.
BUGFIX Hard fallback to en_US.js in Requirements

Ideally we'll have region-less locales like en.js as fallbacks,
but for now this prevents the UI becoming non-operational
e.g. when both i18n::set_locale() and i18n::default_locale()
are set to 'en_NZ'.
NEW Restrict upload abilities in UploadField
NEW Global default config for UploadField
Merge pull request #1087 from chillu/pulls/dataobject-get-join

API Removed non-functional $join argument from DataObject::get() and oth...
Fixed preview overlay showing by default

This was a regression made visible by the recent change to
enforce dimensions on this overlay, which in turn visualizes
the blocked/unblocked states of the preview (see fc6d6ffad)
Show label-less input#file field in UploadField with canUpload=false

The field carries the configuration, and some non-upload functionality
like "attach files" still relies on the fileupload jQuery plugin
being initialized.