Add a page to view an overview of companies

[ConcernedHobbit]

Add a page to view an overview of companies and their job postings. The page should show the list and count of job postings (expired, active and total) per each company.

Issue ported from https://github.com/TKOaly/jobs-service/issues/6

Additionally, users could have a view where especially sponsors can introduce themselves.

[dogamak]

Currently admins have a very rudimentary company listing view. I wasn't sure about the usefulness of such view for the end user, so I left it be for now.

Users can however navigate to a company details page by clicking the company name in the posts. That page would be suitable for some ad-copy about the company.

[ConcernedHobbit]

Maybe we could have a tab for "Sponsors" that has a listing of sponsor companies with ad copy, and users need to navigate to other companies a less visible way?

Search could also have two result panes; job postings and companies (if any match search parameters)