tags: fix for problem with missing javascript

Signed-off-by: Eirini Psallida <eirini.psallida@cern.ch>
Something like [`BlueprintAutoDiscoveryRegistry`](http://flask-registry.readthedocs.org/en/latest/api.html?highlight=blueprint#flask_registry.registries.appdiscovery.BlueprintAutoDiscoveryRegistry) for bundles could be a good fit. (Assuming it doesn't add to much weight to the bootstraping time  #1462)

```python
# -*- coding: utf-8 -*-
# invenio.modules.search or
# invenio.modules.search.bundles
from webassets import Bundle

js = Bundle(
    "js/search/search_parser.js",
    "js/search/typeahead.js",
    "js/search/default_typeahead_configuration.js",
    "js/search/facet.js",
    "js/bootstrap-select.js",
    filters="uglifyjs",
    output="50-search.js"
)
css = Bundle(...)