Simplify Git platform config settings and links

[medubelko]

Background

In #74 we implement a new edit button, which supports both Launchpad and GitHub. It's added through a source_edit_link config setting from Furo, which is redundant because we already have github_url.

Request

• Rename the github_url config setting to repo_url.
• Handle the source_edit_link config setting with behaviour based on the value of repo_url.
• Hide source_edit_link from users.
• Account for Launchpad links in the feedback button.
• Port the JS responsible for the feedback button into Jinja.