Wait for CSS to be loaded before replacing the Body

[manuelpuyol]

Related to #613

As suggested in #613 (comment), we can listen for load and error events in stylesheets to make sure they are loaded before continuing a render.

Here I'm updating how PageRenderer adds new stylesheets to the document head, adding listeners to those events and waiting them to fire before replacing the body.

[dhh]

What would the failure state be if those assets fail to load (bad CDN or whatever)? How long would we wait for them to load? Do we need a timeout?

[manuelpuyol]

What would the failure state be if those assets fail to load (bad CDN or whatever)? How long would we wait for them to load? Do we need a timeout?

good shout, I've added a timeout as @seanpdoyle suggested

[manuelpuyol]

@seanpdoyle @rik what do you think of the changes now? I've implemented your suggestions :)

[dhh]

Do we have adequate test coverage for this?

[manuelpuyol]

Do we have adequate test coverage for this?

added tests in 3171b0a

[manuelpuyol]

@dhh this should be ready for some 👀