Deprecate OutputPage::readOnlyPage()

Also, as this method is never called with an argument in any Gerrit-hosted
extension, shortened it to just `throw new ReadOnlyError;` on the assumption
that the removed portion was only left in for EditPage.

Change-Id: Icc2fc166b155eac548dfd5f3e67b0b1f92ef90d3
Deprecate OutputPage::rateLimited()

Change-Id: I140e6af9dde090b9f8611e7b53a650061f2e84eb
Merge "Deprecate OutputPage::readOnlyPage()"
Merge "Deprecate OutputPage::rateLimited()"