server: Libervia pages: - introduce new LiberviaPage which are easy to create web pages tailored for SàT architecture. They are expected to be the cornerstone of the future of Libervia. - webpages paths reflected by files paths in src/pages directory (directory should be changeable in the future) - if a pages subdirectory as a page_meta.py file, it is a webpage, same for sub-subdirectories and so on - page_meta.py variable are used to instanciate the resource. Check __init__ docstring for details - access is set with a page_meta.py variable (only public for now), and subdirectories access are restricted by parent directory ones. - callables in page_meta.py get 2 arguments: the LiberviaPage resource instance, and the request - callables there can use Deferred (or not) - LiberviaPage has a couple of helper method to e.g. parse URL or return an error