global: deprecate package

[jirikuncar]

Do we have a common way to deprecate packages?

[tiborsimko]

So far we did only gentle deprecation in the docs:

• inveniosoftware-attic/flask-ratelimiter@c54b7a4

We could start with that, and add more aggressive deprecation warnings to the code itself, once we are approaching Invenio v3.0 beta.