Add entry_before_display hook

See https://github.com/FreshRSS/FreshRSS/issues/252
Fix entry_before_insert hook

The hook must be called also in:
- feedController->addAction()
- importExportController->importJson()

See https://github.com/FreshRSS/FreshRSS/issues/252
Prepare better organization of view files for exts

View files must be well-splitted to simplify work for extensions.

See https://github.com/FreshRSS/FreshRSS/issues/252
Fix View files inclusion.

Now, each part of the view (layout, partials, helpers, views) is included based
on the $base_pathnames attribute.
Extensions can now override all of these files.

See https://github.com/FreshRSS/FreshRSS/issues/252
Load user extensions after all the global inits

See https://github.com/FreshRSS/FreshRSS/issues/252
Add behaviour to configure action (extensions)

- Put extension configure view in dir_ext/configure.phtml
- Handle POST action in Extension->handleConfigureAction() method

See https://github.com/FreshRSS/FreshRSS/issues/252