Implement posthog_dashboards submodule

@lrwebks please always document the latest status here!!! Otherwise, we'll have no idea where you've come to a standstill!

@grevil, the latest status of my work: I was struggling quite a bit with the AJAX integration when it came to dynamically adding / removing items from the settings form. After a chat with @anybody about it, we decided that my time was currently better spent on different tasks.
So, as of now, this task is open to anyone again, but it is likely that I will return to it if the priority of this rises in the future.