Remember page state

[krystian8207]

Previously all the pages were initially hidden and shown on bookmark change.
This solution have an important disadvantage:

• hidden page inputs are not bind,
• you need to use suspendWhenHidden = FALSE for all the inputs in order to update them.
  We decided to hide all the pages with visibility: hidden.
  This forces all the pages to be rendered during runtime, but between page state is remembered and not rendered again.

What's more the API is more consistent now. You should use router$ui instead of router_ui() and router$server(input, output, session) instead of router(input, output, session).

[dokato]

I really like these changes. They solve a lot of issues. Please address outstanding minor comments.

Also, from my tests on internal server, looks like it solves #68. I was thinking of introducing backward compatibility with router and router_ui calls, but I can see that this would be difficult in the current setting, so maybe we can ignore.

[dokato]

I tested locally and on R Connect, looks good now 👍