1. An exception would be raised if the filter was called with an invalid
   URI. Mainly because we weren't catching the `Addressable` exception.

2. This commit fixes it and adds a spec for the filter.

Original Comments
=================

- Linking behaves as per rules documented here:
  https://gitlab.com/gitlab-org/gitlab-ce/blob/16568-document-wiki-linking-behavior/doc/markdown/wiki.md
- All links (to other wiki pages) are rewritten to be at the level of
  the app root. We can't use links relative to the current
  page ('./foo', 'foo', '../foo'), because they won't work in the
  markdown preview, where the current page is suffixed with `/edit`
- Move existing `WikiLinkFilter` specs to `WikiPipeline` spec. It makes
  sense to run these tests on the combined output of the pipeline,
  rather than a single filter, since we can catch issues with
  conflicting filters.
- Add more tests to cover the new linking

@rymai's Review
===============

- Classes nested under `WikiLinkFilter` should declare `WikiLinkFilter`'s
  inherit, so nothing changes if the nested class is loaded first.
- Add a blank line after a guard clause
- Use keyword arguments for the `Rewriter` constructor
- Invert a condition - use `if` instead of `unless`
- Inline a `let` in `WikiPipeline` spec - it was only used in a single place
- Change out of date spec names
- Add a comment for every rewrite rule in `Rewriter`

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb

- And fix behavior for non-file hierarchical links.