Only allow submissions/edits for JRubyConf EU

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Update copy to reflect different JRubyConf EU deadline

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Hide an edit button for non-JRubyConf proposals

Signed-off-by: Alex Coles <alex@alexbcoles.com>