[JENKINS-18723] Calculate a variable only once.

Fixes a major performance bottleneck described in [JENKINS-18723](https://issues.jenkins-ci.org/browse/JENKINS-18723). 

*Original scope*:

To be short: instead of calculating a current page url for EVERY job in the view which was triggering the calculation of ACLs for all the jobs in Jenkins for a lot of times (that was _actually_ happening if we debug the code), now we do not calculate the current page link at all - any browser can do this for us. Therefore we just specify a job link and browser will prefix it with current page.

That really boosts performance especially on a large set of jobs and views which we have at work. 

*Reduced scope*:

Avoid doubling calculation.