Enable option for relative links on shared pages

[mikemonteith]

Add a middleware to make wagtail's pageurl templatetag create relative
links. Without this middleware, hyperlinks on sharing.mysite.com will
take a user to mysite.com.

Our reviewers want to review multiple draft pages at once, so the page links should keep a user on the sharing domain.

Additions

Added a middleware which will enable page links to keep the sharing domain

Testing

Middleware unit tests

Review

• @user

Preview this PR without the whitespace changes

Checklist

• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Code follows the standards laid out in the front end playbook
• Passes all existing automated tests
• New functions include new tests
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged
• Visually tested in supported browsers and devices
• Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)

[chosak]

@mikemonteith thank you for the PR! This is an interesting idea. Can you confirm how many Sites you're using?

As far as I can tell, Wagtail's SiteMiddleware will only ever set request.site to be None if there are either no Sites at all or if there are multiple Sites but the request doesn't match any of them. Does that latter case describe your setup? In the single Site case then request.site should already be set properly.