Create a URL which initiates a library search

[jsnshrmn]

Description

Adds:

• an intentionally minimal page with an intermediary form at /search/?q=<search terms> that sends users from stable urls on-wiki to searches in EDS.
• an eligible editors only view mixin that raises permission denied for users without an editor and redirects ineligible editors to my_library, which explains eligibility

Updates:

• the referring url policy to be in django instead of nginx. the header was getting added twice and it was causing intermittent problems during testing
• Drops the from_homepage query parameter once our auth check is done using it

Rationale

For use in the Find Sources template

Phabricator Ticket

https://phabricator.wikimedia.org/T294919

How Has This Been Tested?

I added unit tests for the new view mixin, since that is reusable and has security implications.
I manually tested:

• multiple locales, including those that need the code manipulated, like pt, with JavaScript enabled and disabled.
• eligible and ineligible editor accounts
• anonymous and logged in users

Screenshots of your changes (if appropriate):

nojs screenshot; the javascript version automatically sends the user to EDS.

Types of changes

What types of changes does your code introduce? Add an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Minor change (fix a typo, add a translation tag, add section to README, etc.)

[suecarmol]

Just a small change before I can merge this

[suecarmol]

Thank you for working on this!