Export SeoAssessor from project index

[hansjovis]

Fixes #1878

Summary

This PR can be summarized in the following changelog entry:

• [not user facing] The SEOAssessor is exported from the analysis index again.

Relevant technical choices:

• This was exported previously I believe but removed in the refactoring to import/export.

Test instructions

This PR can be tested by following these steps:

• Install yoastseo through NPM
• Try to import { SeoAssessor } from 'yoastseo';
• See that webpack happily compiles

Fixes #1878

[hansjovis]

Made this PR to trigger a Travis build for #2079 .
Build passes so this can be closed.