Simplify the User database model

[sphuber]

The current User entity is mostly used to add the metadata of the owner to a Node. However, it includes various attributes related to using the instance as way to authenticate and authorize. This was originally added because the base user implementation provided by Django was used, which requires these additional fields. If Django can be used with a simplified model, this would be better. The only attributes we need:

• id
• email
• first name
• last name
• institution

[sphuber]

Question @ltalirz and @giovannipizzi : we didn't discuss if having first and last name as separate attributes is necessary. These things traditionally can be pitfalls. Not all people have a name that falls into this category. Take my own name for example. I would need to include my middle name to match my academic publishing name. Where do I put it? This is also the reason why git just has "name". I propose we simplify further to just a single attribute name.

[giovannipizzi]

ok for me

[ltalirz]

I'm ok with the change (name = first name + last name) in general but I would hesitate to include this in 1.0.
I don't think it's a necessary change and it might break some things (e.g. yml files for verdi quicksetup).
Anyhow, I'm open to discussion.

[CasperWA]

Note for import/export:
This will not change anything for import/export, since only: "last_name", "first_name", "institution", and "email" are (and have been) exported.

[sphuber]

Decision has been made to merge first_name and last_name into single name column, but since this will introduce breaking changes, we will mark this for release v2.0.0 see #2945