BASE_URL setting should be renamed

The BASE_URL setting used by wagtailadmin for generating URLs to put in emails is a bit inconsistent with the rest of Wagtail’s settings, which are all prefixed with the module name.

I think this should be renamed to WAGTAILADMIN_BASE_URL (with backwards compatibilty for the previous setting).

[mx-moth]

May I also suggest:

1. Documenting the WAGTAILADMIN_BASE_URL setting. BASE_URL is currently not documented.
2. If WAGTAILADMIN_BASE_URL is not set, falling back on request.site.root_url, then falling back to building a URL from the request HOST_NAME.

[lb-]

Hey @sandilsranasinghe here are some answers.

* there are some usages of `settings.BASE_URL` in some html templates. Should I replace those with the tag as base_url or just change them to `settings.WAGTAILADMIN_BASE_URL`

• adding a tag is probably the best for in template usage.

* by pull out to a generic util do you mean add a util in admin and then use that util in the base_url_setting tag?

• yes, there should be one place (util) that does all the fallback behaviour+ read settings for this as there are two fallbacks

* for the reference should I add documentation under the topic Email Notifications or just create a new topic for this

• good question, not sure but a new section makes sense I think. As you have done in your PR looks good