New wrappers Minz_Request::good() and bad()

1. Set a notification message in session variable
2. Redirect to a specific url

First use in importExportController.php

See https://github.com/marienfressinaud/FreshRSS/conversations/576
Use REQUEST_URI instead of SCRIPT_NAME for cookies

See https://github.com/marienfressinaud/FreshRSS/commit/eceb7756cfcf117c2a18984291181a84697ed3cd#commitcomment-7345438
Fix a Minz_Session TODO

Not use additional variable to manipulate session variables
Fix coding style
Fix coding style of Minz_Request
Fix missing REQUEST_METHOD

Useful when executing actualize_script.php
Move htmlspecialchars_utf8 from Request to Helper

And remove html_chars_utf8 to use htmlspecialchars_utf8 instead in
importExportController
Coding style in Minz_Helper
Merge branch 'dev' of github.com:marienfressinaud/FreshRSS into dev