Improve readme markup,  fixes #7455

##### 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](https://gitlab.dds.ec/public/projects/bob.rudis/iptools)

##### What are the relevant issue numbers?

This PR fixes 
* #7455.
* http://stackoverflow.com/questions/25169393/gitlab-rendering-the-wrong-readme-in-project-public-views/25193268#25193268

It's also related to #7023 and https://gitlab.com/gitlab-org/gitlab-ce/issues/296
/cc @jacobvosmaer 

@jvanbaarsen Could you review?
Thanks for the suggestion. But I would need to add these methods to 3 helpers (projects_helper, snippets_helper, tree_helper). Is that right?