Add new logged-out redirect option for `/bookmarks` pages

[karlhorky]

Similar to #1327

Currently, when copying a logged-in URL (eg. https://demo.linkding.link/bookmarks?q=%23china) to a contact who is not signed up for your Linkding instance, it redirects to a login page:

https://demo.linkding.link/login/?next=/bookmarks%3Fq%3D%2523china

This happens even if the Landing page option is set to Shared Bookmarks in Global settings:

Landing page

[ Shared Bookmarks ▼ ]

The page that unauthenticated users are redirected to when accessing the root URL.

In almost all cases, the other contact will not want to sign up just to view the bookmarks under that tag.

I would propose a (default off) setting for configuring redirects for /bookmarks URLs for logged-out users:

Bookmarks page

[ Shared Bookmarks ▼ ]

The page that unauthenticated users are redirected to when accessing the bookmarks pages.

This could also retain the query string when redirecting logged-out users to the Shared Bookmarks option for Bookmarks page (similar to issues #1327 and #1336), so that an example redirect would look like:

before redirect:
https://demo.linkding.link/bookmarks?q=%23china

after redirect:
https://demo.linkding.link/bookmarks/shared?q=%23china