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?
instead of using `Gitlab::MarkdownHelper.gitlab_markdown?` in several places I recommend you create a helper method `gitlab_markdown?` that calls `Gitlab::MarkdownHelper` internally. In this case we don't need to change views and helpers in other places.

Ex. this files only change is your new class name: 

* app/views/projects/blob/_text.html.haml 
* app/views/projects/edit_tree/preview.html.haml
* app/views/shared/snippets/_blob.html.haml