tec: Allow to change CSP header from controllers

For an extension, I needed to call a script from an external domain.
Unfortunately, the CSP headers didn't allow this domain and I had to
patch manually the FreshRSS FrontController for my extension. It's
obviously not a long-term solution since it has nothing to do in the
core of FRSS, and I don't want to apply this patch manually at each
update.

With this patch, I allow changing the CSP header from inside the
controller actions. It allows extensions to modify headers. It's also an
opportunity to remove a bit of code from the FrontController. I wasn't
happy with the previous implementation anyhow.

Reference: https://github.com/flusio/xExtension-Flus/commit/ed12d56#diff-ff12e33ed31b23bda327499fa6e84eccR143

app/Controllers/statsController.php

@@ -15,6 +15,11 @@ class FreshRSS_stats_Controller extends Minz_ActionController {
 			Minz_Error::error(403);
 		}
 
+		$this->_csp([
+			'default-src' => "'self'",
+			'style-src' => "'self' 'unsafe-inline'",
+		]);
+
 		Minz_View::prependTitle(_t('admin.stats.title') . ' · ');
 	}