Allow moving wiki pages from the UI

Gemfile

app/assets/stylesheets/pages/wiki.scss

app/controllers/projects/wikis_controller.rb

app/helpers/wiki_helper.rb

app/models/project_wiki.rb

app/models/wiki_page.rb

app/views/projects/wikis/_form.html.haml

app/views/projects/wikis/edit.html.haml

...ogs/unreleased/fj-37273-moving-wiki-pages-from-the-ui.yml

config/initializers/gollum.rb

doc/user/project/wiki/img/wiki_move_page_1.png

doc/user/project/wiki/img/wiki_move_page_2.png

doc/user/project/wiki/index.md

lib/gitlab/git/wiki.rb

locale/gitlab.pot

spec/features/projects/wiki/user_updates_wiki_page_spec.rb

spec/features/projects/wiki/user_views_wiki_page_spec.rb

spec/lib/gitlab/git/wiki_spec.rb

spec/models/wiki_page_spec.rb