This commit introduces a new use_polymorphic_bookmarks site setting
that is default false and hidden, that will be used to help continuous
development of polymorphic bookmarks. This setting **should not** be
enabled anywhere in production yet, it is purely for local development.
This commit uses the setting to enable create/update/delete actions
for polymorphic bookmarks on the server and client side. The bookmark
interactions on topics/posts are all usable. Listing, searching,
sending bookmark reminders, and other edge cases will be handled
in subsequent PRs.
Comprehensive UI tests will be added in the final PR -- we already
have them for regular bookmarks, so it will just be a matter of
changing them to be for polymorphic bookmarks.
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