Add extension hooks for static file writing

Allowing for project level customisation of the environment within which
the static content is obtained before it is written.

For example allowing third party modules to affect whether their
middleware is active or not, which may otherwise prevent accurate
content retrieval.

src/ErrorPage.php

@@ -322,6 +322,7 @@ public function writeStaticPage()
         // Run the page (reset the theme, it might've been disabled by LeftAndMain::init())
         $originalThemes = SSViewer::get_themes();
         try {
+            $this->extend('onBeforeStaticWrite');
             // Restore front-end themes from config
             $themes = SSViewer::config()->get('themes') ?: $originalThemes;
             SSViewer::set_themes($themes);
@@ -336,6 +337,7 @@ public function writeStaticPage()
         } finally {
             // Restore themes
             SSViewer::set_themes($originalThemes);
+            $this->extend('onAfterStaticWrite');
         }
 
         // Make sure we have content to save