Work around server reloading issues

[kamilzyla]

Changes

Closes #157. This was a really tricky bug and I still don't fully understand why it happens. My comments in the issue detail the investigation; this one in particular provides a minimal example which explains the changes in this PR.

In short, it seems that two things must be true about the server passed to shinyApp() for reloading to work properly:

1. It must be defined using curly braces (function(input, output) { module_server("app") } and not function(input, output) module_server("app")).
2. It must be evaluated (defined, created) in a file sourced with keep.source = TRUE.

How to test

Initialize a fresh project and run it with shiny::runApp(). Add some changes in the server of main.R, e.g. change the "Hello!" string. It should be sufficient to touch app.R (update its timestamp) and refresh the page in the browser to see the changes.

[kamilzyla]

The issue persists, I will investigate further.

[jakubnowicki]

Great that you found a way to make it work! Still, since it is a hacky solution, we need to make sure that we keep checking if a more elegant solution is available (e.g. after a new version of shiny lands).