global: use SemanticUI by default

Cookiecutter-Invenio-Instance should by default use SemanticUI. This means that it should somewhere define in the configuration:

```python
APP_THEME = ['semanticui']
```

To-do:
- when `invenio-theme` is released a stable, not anymore `a1`, then revert the `/scripts/bootstrap` to use Pipenv instead of manual installation. Currently there is an issue installing pre-release versions with Pipenv.

Start by reviewing merged pull request #227 and the /scripts/bootstrap path named in the issue to determine what remains. Locate where the instance configuration defines APP_THEME and verify that the default is semanticui; once invenio-theme is stable, completion also requires restoring Pipenv usage in the bootstrap script.

Written by the indexing model from the issue text.