search: fix decoding Unicode is not supported.

**Problem**

Visit `/admin/websearch/collection/view/Articles%20%26%20Preprints`
```
Traceback (most recent call last):
  File "/home/vagrant/.virtualenvs/pu/lib/python2.7/site-packages/flask/app.py", line 1836, in __call__
    return self.wsgi_app(environ, start_response)
  File "/home/vagrant/invenio/invenio/ext/legacy/__init__.py", line 61, in __call__
    response = self.app.handle_exception(e)
  File "/home/vagrant/.virtualenvs/pu/lib/python2.7/site-packages/flask_restful/__init__.py", line 258, in error_router
    return original_handler(e)
  File "/home/vagrant/.virtualenvs/pu/lib/python2.7/site-packages/flask/app.py", line 1403, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/home/vagrant/invenio/invenio/ext/legacy/__init__.py", line 57, in __call__
    response = self.app.full_dispatch_request()
  File "/home/vagrant/.virtualenvs/pu/lib/python2.7/site-packages/flask/app.py", line 1477, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/home/vagrant/.virtualenvs/pu/lib/python2.7/site-packages/flask_restful/__init__.py", line 258, in error_router
    return original_handler(e)
  File "/home/vagrant/.virtualenvs/pu/lib/python2.7/site-packages/flask/app.py", line 1381, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/home/vagrant/.virtualenvs/pu/lib/python2.7/site-packages/flask/app.py", line 1475, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/vagrant/.virtualenvs/pu/lib/python2.7/site-packages/flask/app.py", line 1461, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/home/vagrant/.virtualenvs/pu/lib/python2.7/site-packages/flask_login.py", line 658, in decorated_view
    return func(*args, **kwargs)
  File "/home/vagrant/.virtualenvs/pu/lib/python2.7/site-packages/flask_principal.py", line 199, in _decorated
    rv = f(*args, **kw)
  File "/home/vagrant/invenio/invenio/modules/search/views/admin.py", line 159, in manage_collection
    TranslationsFormFilled = TranslationsForm(language_list_long(), translations)
  File "/home/vagrant/invenio/invenio/modules/search/admin_forms.py", line 46, in TranslationsForm
    setattr( _TranslationsForm, lang,  TextField(_(unicode(lang_long,"utf-8")), default = \
TypeError: decoding Unicode is not supported
```

**Solution**

the input string is already a `unicode` object, so there is no need to decode it.
```diff
- unicode(lang_long,"utf-8")
+ lang_long
```