Support Wiki with UTF-8 page name

## What does this MR do?

Support Wiki with UTF-8 page name.

See https://github.com/gollum/gollum/pull/929

## Why was this MR needed?

Relax constraints for wiki slug in aac65984.

It allows to create a wiki with UTF-8 name, but creating a wiki with UTF-8 name causes 500 error.

Creating a wiki with UTF-8 name once, then creating or updating wiki with ascii name also cause 500 error i.e. no one can create and update any wiki pages in the project.

## Workaround

1. Go to `https://DOMAIN/repo/wikis/git_access` -> this page display the link to git clone
2. Clone the wiki repo
3. Find the page with UTF-8 name
4. Rename or Delete these files
5. Commit and push

## What are the relevant issue numbers?

- #13979
- #13891
- #13698
- #13603
- #13317
- #12906
- #12825
- #10945 

## Todo

- [x] Waiting for 'gollum-rugged_adapter' that support rugged v0.24.0 release

'gollum-rugged_adapter' gem doesn't allow to install rugged v0.24.0 (it's still beta version),
but 'gitlab_git' gem depends on rugged v0.24.0b13.

So it can't install both 'gollum-rugged_adapter' and 'gitlab_git' now.


See merge request !2999