When we build and send emails using MessageBuilder and Email::Sender
we add custom headers defined in SiteSetting.email_custom_headers.
However this was causing errors in cases where the custom headers
defined a header that we already specify in outbound emails (e.g.
the Precedence: list header for topic/post emails).
This commit makes it so we always use the header value defined in Discourse
core if there is a duplicate, discarding the custom header value
from the site setting.
cf. https://meta.discourse.org/t/email-notifications-fail-if-duplicate-headers-exist/222960/14
Zeitwerk simplifies working with dependencies in dev and makes it easier reloading class chains.
We no longer need to use Rails "require_dependency" anywhere and instead can just use standard
Ruby patterns to require files.
This is a far reaching change and we expect some followups here.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
