[FIXED JENKINS-23339] - Introduce the validated Jenkins::getActiveInstance() method

Currently, the <code>jenkins.model.Jenkins::getInstance()</code> method is marked as <code>@CheckForNull</code>, but the most of Jenkins code does not actually check the return value. It leads to tons of static analysis errors, hence it is hard to analyze the code.

This change:
* Introduces new <code>@Nonnull jenkins.model.Jenkins::getActiveInstance()</code> method
* Converts all unchecked calls within the core to the new method call

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>