Improve cold start times

[efcaguab]

The app is relatively slow to start when the containers are cold. We should do a few adjustments to improve this situation.

• Install the package in the docker image instead of loading it during the app initialisation (should shave off about half a second)
• Reduce dependencies: apexcharter has an unneeded dependency to ggplot2 which add a few extra ones. Pkgload could also be removed if we install the package in the docker image and the dependency on golem (which in turn imports a lot of packages) could also be removed. Not sure how much improvement could be achieved doing this but at least the code will be more maintainable.

Alternatives like pinging the app with a CRON job could also be tried in the future.

[efcaguab]

Removed pkgload in 56652f3