Prefer get_server_url() in notifications templates

Review Request #8889 — Created April 7, 2017 and submitted — Latest diff uploaded

Information

Review Board
release-3.0.x

Reviewers

We build the site URL manually in the majority of the e-mail templates.
However, we have a utility method which does this exact logic, so we can
just pass the result of that in instead.

Additionally, the API token preference URL has been refactored to use
the build_server_url().

Previewed e-mails. They looked correct and links still worked.