renders local login form if and only if you're using it

[felkerch]

Description of Changes

Updates the login view to render the local login form only if it's one of the auth providers.

Steps to Test

1. Local setup:

• In amphora-auth: npm i && npm link
• In amphora: npm i && npm link && npm link amphora-auth
• In your app: npm link amphora

2. To test, modify your amphora instantation:

const providers = ['google', 'local'];
amphora({
      app,
      renderers,
      providers,
      sessionStore,
      storage,
      eventBus,
      plugins: [amphoraSearchPlugin, amphoraSchedule],
      bootstrap: bootstrapOnStart && isBootstrapProcess
    });

2. Add 'local' to this list and verify that you see the login form. Remove it and verify that you don't.

[mattoberle]

This looks good to me!

I added a test locally to index.test.js that looked like this and removed the .not

    it('returns useLocalAuth=true when the local provider is enabled', function () {
      fn({ path: '' }, ['local'])({ flash: _noop }, mockRes);                    
      fn({ path: '' }, [])({ flash: _noop }, mockRes);                           
      expect(mockRes.send.mock.calls[0]).not.toEqual(mockRes.send.mock.calls[1]);
    });

The diff in the rendered template is what I'd expect.

[mattoberle]

@felkerch your PR is merged and I'm going to release a couple of versions.

The v1.3.x versions of amphora-auth implement the "secure" flag on the session cookie.
When running Clay behind a proxy this may require some additional work.

From what I understand, not everyone is ready to adopt that flag yet.

As a result, I'm going to release v1.2.1 which backports this PR but excludes the secure cookie change.
That release will be cut from the v1.2.x branch.

I'm also going to release v1.3.2 which will be cut from the master branch.

[felkerch]

Thanks @mattoberle!