Fixed Tab floating in CMS (fixes #1778)

Removed inline-block from header elements which seems
to cause the issues of tabs stacking on top of each other.
Replaced header background to work on multiline scenarios
Added bottom margin to breadcrumbs so they don't look cramped
when tabs are floated below them.

See https://github.com/silverstripe/silverstripe-framework/pull/1863,
https://github.com/silverstripe/silverstripe-framework/issues/1943,
https://groups.google.com/forum/?fromgroups#!topic/silverstripe-dev/c-W-ZxDlDuA,
http://open.silverstripe.org/ticket/6882
Fixes a translation issue where a parameter isn't replaced.

Whenever File::Validate() failed due to an illegal file extension, it printed the translated message without replacing the parameter {extensions}.
Merge pull request #2090 from chillu/pulls/tabs-float

Fixed Tab floating in CMS (fixes #1778)
Merge pull request #2095 from CheeseSucker/patch-1

Fixed a translation issue for File.INVALIDEXCEPTION
Fixed HtmlEditorField.js editor instanciation

Wasn't parsing data-editor attrs correctly.
Using proper instances of the underlying editor now,
instead of relying on "active editor" states which get
wonky once they lose focus (e.g. when opening a dialog).