Automate pip requirements

[chrisjsewell]

Currently, they are specified directly in defaults/main.yml per plugin, split into requirements that should be installed in the aiida or jupyter virtual environment.

Although the aiida requirements are specified in the actual app repositories, they are not trivial to resolve, because they use different files to specify these requirements, and not always specified directly (using functions instead)
See for example:

• https://github.com/aiidalab/aiidalab-widgets-base/blob/v1.0.0b14/metadata.json
• https://github.com/aiidalab/aiidalab-qe/blob/master/metadata/templates/metadata.json.in

Instead, they should be read from https://aiidalab.materialscloud.org/appsdata/apps_meta.json
Note this also requires fixing aiidalab/aiidalab-home#51

For the jupyter environment, these requirements are not yet directly specified anywhere.
The closest is that the quantum-espresso app specifies: https://github.com/aiidalab/aiidalab-qe/blob/master/jupyter-config/nbconfig/notebook.d/aiidalab.json (this is how the current requirements were derived for both that and widget-base which shares the same structure viewer)

cc @csadorf

[chrisjsewell]

This may be of relevance: https://github.com/aiidalab/ansible-role-aiidalab-server/blob/master/files/merge_app_metadata.py