Implement optimisations to prevent the notebook fan from turning on so much

[leo-holanda]

I think that one of the reasons that this app is making the notebook fan turn on is because there are lots of things that should run just once and it's running multiple times.

In the process of mapping a job posting, the title and description is parsed multiple times when just one is enough. When this process is multiplied by the number of jobs and considering the title and description string sizes, the related-terms maps iteration and so on, it surely takes a toll on the computer resources.

So instead of parsing it for every piece of data that is meant to be extracted, just parse it once and then extract all data in one pass.

Also, the ranks in the all-overview are being remade everytime the user returns to the stats route when they should be remade just when the selected job collections change.

Think more about possible optimisations.