Links between modules for Javadocs on developer.atlassian.com are incorrect or broken

When linking from, for example, the spi Javadocs to the api Javadocs, links are often missing or incorrectly structured, or, if the link is present, it's broken.

• Links between modules still refer to "stash" where they should refer to "bitbucket-server"
• Many places where the Javadocs in the source use {@link} are not rendered as links at all
• Some links have incorrect packages, or have "null" in place of class names

Additionally, links to JDK classes like String are still referencing Java 6, even though Java 8 is required. That means links to newer classes, like Optional, are not rendered.