Pages copy: create tags in correct language.

During page copy, the tags were not created in the target language.
This change sets the target language when saving tags, and also makes
sure that BackendTagsModel::getURL() uses the correct language, so that
the slug for the tag does not have any unnecessary '-2'-like suffix.
Remove trailing tab
Tags BE: BackendTagsModel::getStartsWith() uses language.

Tags are language dependent, but the method for auto completing tags did
not take the language of tags into account.  This change does use the
language for retrieving tags.
FIXED: Form_builder Reply-To field flag not saved (thus not used)
Cleanup
Merge pull request #580 from forkcms/fix-tag-autocomplete

Tags BE: BackendTagsModel::getStartsWith() uses language.
Merge pull request #579 from WouterSioen/patch-12

Remove trailing tab
Merge pull request #578 from forkcms/fix-pages-copy

Pages copy: create tags in correct language. (REVIEW FIRST)