"update" should symlink things "install" does

Currently `invenio-cli install` will also symlink the `invenio.cfg`, `templates`, and `app_data` folders (see [here](https://github.com/inveniosoftware/invenio-cli/blob/335f5d12f4b7b80362ccff999ae52d3b40bc1add/invenio_cli/helpers/commands.py)). In case someone e.g. wipes the `var/instance` folder, it should still be possible to avoid Python dependencies installation and selectively just symlink all necessary folders/files.

This issue could be also partially addressed by #128.

Start in invenio_cli/helpers/commands.py, at the install symlink handling referenced by the issue, and compare it with the update command. Make update symlink invenio.cfg, templates, and app_data without installing Python dependencies; verify the behavior after var/instance is wiped.

Written by the indexing model from the issue text.