Optional Company URL on the front page in the 'Great Companies' section.

As a portfolio author,
I want to add a company without a URL to the front page (to the "Great Companies I've Worked with" section)
because not every company has (still) a page.

## Acceptance Criteria
1. Company URL is optional.
2. The Company logo with no provided URL doesn't have a link but is shown in the same way.