Add a Through model between Users and Maintainer/Author accounts

Broken off from #667

> Future imports of authors and maintainers as Users-- we should consider how future releases will also contain libraries.json files that don't map directly to user accounts (people use multiple emails, or are part of multiple orgs depending on the libs they work in etc.). We already save the libraries.json data on the LibraryVersion record, which will be helpful in matching. But a future enhancement might be allowing a user to have multiple Author and Maintainer records, or adding field(s) to the User model to capture what's stored in the libraries.json files of the libs they work with. Something to make matching the libraries.json to the real Users easier.

This was in the context of this problem:

- We create stub "unclaimed" users when we create new libraries and library-versions based on what's in the `libraries.json` file
- We then let users "claim" those accounts (see #667) when they register if they use the same email
- At that point, since they own the account, they can change their email address
- The next time the update runs, we won't be able to match them based on email anymore, so we will create a new stub user record for them
- What we need is a Through model between User and Contributor. The "Contributor" model needs a text field and replaces the "User" link on the ManyToMany for Library and LibraryVersion. Text field contains whatever is in the `libraries.json` file. Doesn't need to be unique -- two Jane Smiths could author two different Boost libraries, or maintain different ones, etc.

## Things to consider

- [ ] What is the new flow for claiming a contributor account?
- [ ] We won't need the `claimed` field on `User` anymore
- [ ] We won't need the overrides of the allauth password reset forms anymore, since we won't need to mark users as claimed?