Add existingSecret option, align postgres PW handling

Hey @bufke,

I wanted to use the Glitchtip Helm chart, but would need some functionality many other charts have before I can do so: The ability to define an existing secret where the application gets environment variables from. This is needed when using akv2k8s, for example.

I tried to make my changes the least intrusive as possible, but on my way I saw that there was an inconsistence between the web, worker and beat deployment regarding the handling of the postgres password, so I quickly aligned those lines, too.

Thank you for your feedback in advance!