Remove view fragment caching for project READMEs

In https://gitlab.com/gitlab-com/support-forum/issues/1407#note_21183148 @stanhu mentioned that we are not only caching the raw Markdown of a README, but also the rendered view fragment. This can lead to race conditions where both caches use different data (as described in https://gitlab.com/gitlab-com/support-forum/issues/1407#note_21265969).

The easiest approach that I can think of is to:

1. Cache rendered Markdown (instead of raw Markdown) and refresh this on every push
2. Remove the view fragment cache

If we use the raw Markdown somewhere we could add an extra method (e.g. markdown_html), cache it, and use that on the project homepage.