Revisit CSS compiler lifecycle

[leandrocp]

In order to generate all the expected rules from all pages, the CSS compiler does fetch templates from loaded pages but the problem with this approach is that those pages are not loaded during the app boot which generates an empty stylesheet, and the old approach would fetch the template from the database but would miss some rules since page templates can be post-processed (the final template stored in ETS differs from the one stored in the DB). Note that we can't just load all modules eagerly, see #330 for more info.

[leandrocp]

Reopening due to a performance issue. Working on it.