Improve markup,  fixes #7455

##### What does this MR do?

This PR renders markup file without markup, if markup fails for some reason. Additional readme is rendered with `simple_format` if there is no proper renderer available.

##### Why was this MR needed?

Currently, if markup fails for some reason, GitLab displays a 500 error.

##### What are the relevant issue numbers?

This PR fixes #7455.

It's also related to #7023 and https://gitlab.com/gitlab-org/gitlab-ce/issues/296

/cc @jacobvosmaer 
Prefer `select` over `find_all`.