Add and edit custom Wiki sidebar support from the UI

Problem to solve

With https://gitlab.com/gitlab-org/gitlab-ce/issues/14995 we are adding an option to add a custom Markdown sidebar navigation. In this first iteration, it's based on the convention to add a _sidebar Wiki page that overwrites the default generated table of content.
We should add an option to add a custom sidebar from the UI, as well.

In addition, there is the open question if besides overwriting the generated pages TOC, adding the custom sidebar, in addition, doesn't make sense as well.

Further details

(Include use cases, benefits, and/or goals)

Proposal

• When a custom sidebar is not configured show a link to create a custom nav which will create a _sidebar page
• When a custom sidebar is configured show a link to edit the _sidebar page

What does success look like, and how can we measure that?

This is resolving UX debt that the custom sidebar feature isn't discoverable or easily editable.

Links / references

https://gitlab.com/gitlab-org/gitlab-ce/issues/14995