Partially addresses #47424.

This moves the logic of detecting special repository files (e.g. a
README or a Koding configuration file) to a single class:
Gitlab::FileDetector. Moving this logic into a single place allows this
to be re-used more easily.

This commit also changes Repository#gitlab_ci_yaml so that its cached
similar to other data (e.g. the Koding configuration file).

Take the first previewable README file as project’s README, otherwise
if none file is available, or we can’t preview any of them, we assume
that project doesn’t have a README file.

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

This is shown at the creation of new issues and new merge requests, when
the repository has a contribution guide file.

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Didn't bother with files in db/, config/, or features/