Improve readme markup, fixes #7455

Created by: Razer6

What does this MR do?

• Render readme markup rendering fails, throw no 500 error
• Render readme we support if there are multiple readmes in repo
• Render readme with simple_format if we don't have a proper render, to preserve style

Why was this MR needed?

• Currently, if markup fails for some reason, GitLab displays a 500 error.
• If we've a readme without a render, readme looks ugly

What are the relevant issue numbers?

This PR fixes

• #7455 (closed).
• http://stackoverflow.com/questions/25169393/gitlab-rendering-the-wrong-readme-in-project-public-views/25193268#25193268

It's also related to #7023 (closed) and https://gitlab.com/gitlab-org/gitlab-ce/issues/296 /cc @jacobvosmaer

@jvanbaarsen Could you review?