ext: arXiv Flask extension database search fix

* Adds an instance wide search parameter prefix to perform the database
  search.

* NOTE: for the search to be performed in the database, the arXiv field
  prefix for the corresponding instance config file should be added to
  invenio.cfg as ARXIV_SEARCH_PREFIX
  (e.g. ARXIV_SEARCH_PREFIX = u'035__a:oai:arXiv.org:').
be careful about brackets:
```python
c = ('a' + None or 'b')
c = ('a' + (None or 'b'))
```