The issue was fixed in 2ba5e62. The spec is going in separately just
so the fix could go in as soon as possible.

- Need to pass in a `page_slug` to the filter, so it can rewrite based
  on the current page (all links are rewritten to the level of the app root).
- The earlier `markdown_preview` endpoint was at the level of the wiki.
  We need to know the current page (for rewriting, as above), so this
  commit moves the endpoint to the level of a wiki page.
- Fix all tests

Resolves #14411.

See merge request !3464

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

All inputs of type `search` will have the gray background
and search icon centered.
Because the search magnifier is a bg image,
I had to hide it with `[value=""]`.
I added a little javascript to make sure each input always has it's
own value.

Since GitHub doesn’t apply these constraints to theirs wiki slug
allowing characters like `,`, `:`, `*`, etc, we need to relax our
constraints or some wiki pages will not be available after they were
imported.

For an example the Devise project have a wiki page with the following
slug: “How To: Add sign_in, sign_out, and sign_up links to your layout
template”

Closes #2104

Closes #1751

Closes #1592

Closes https://github.com/gitlabhq/gitlabhq/issues/9399