Refactor RedirectService::addRedirect()

[mmikkel]

Refactors how existing redirects are handled when adding new redirects (or editing existing ones):

• Existing redirects are never updated, only deleted, if the redirect being added (or edited) has the same siteId (or a null siteId) as the existing one
• If an existing redirect has the same sourceUrl, but a different siteId from the redirect being saved, it's left alone
• In querying for existing or conflicting redirects when saving an edited redirect, the id param is added to the queries

This solves some unexpected behaviours when adding new redirects or editing existing ones.