tests: add webpack build tests

[lnielsen]

The tests should build the webpack bundle. Currently, with a passing travis build we do not know if the webpack builds or doesn't build. Furthermore the build is dependent on the Webpack config defined by Invenio-Assets (Invenio-Assets is already in requirements-devel.txt thus devel tests will pickup latest webpack config to check if there's any issues).

Proposal:

• Add a test fixture to pytest-invenio that easily allows testing of themed bundles (basically the fixture/tests should be parameterised to run for each theme defined in APP_THEME).
• Add documentation to Invenio-Assets on how to test a Webpack bundle.
• Add test to build the bundle here in Invenio-Search-UI