Add flag to optionally limit redirects to url_prefix

[Lukewh]

Done

By default, redirects are very restrictive, this change makes things more open by default, which is fine in 99% of our use-cases.

QA

• You'll need to include this branch in your project.
• Add a couple of redirects to your discourse docs:
  1. /docs/test -> /docs
  2. /docs/test -> https://github.com
• With your Docs initialised with url_prefix="/docs":
  • limit_redirects_to_url_prefix set to True redirect 1. should work and redirect 2. should not.
  • limit_redirects_to_url_prefix unset (or set to False), both redirects should work

Fixes https://warthogs.atlassian.net/browse/WD-2871

[sparkiegeek]

conflicts with main now

[Lukewh]

I'll fix the tests later - looks like I need to update new ones

[anthonydillon]

@Lukewh is this still valid?