Implement Web scraping "HTML + XPath" (#4220)

* More PHP type hints for Fever
Follow-up of https://github.com/FreshRSS/FreshRSS/pull/4201
Related to https://github.com/FreshRSS/FreshRSS/issues/4200

* Detail

* Draft

* Progress

* More draft

* Fix thumbnail PHP type hint
https://github.com/FreshRSS/FreshRSS/issues/4215

* More types

* A bit more

* Refactor FreshRSS_Entry::fromArray

* Progress

* Starts to work

* Categories

* Fonctional

* Layout update

* Fix relative URLs

* Cache system

* Forgotten files

* Remove a debug line

* Automatic form validation of XPath expressions

* data-leave-validation

* Fix reload action

* Simpler examples

* Fix column type for PostgreSQL

* Enforce HTTP encoding

* Readme

* Fix get full content

* target="_blank"

* gitignore

* htmlspecialchars_utf8

* Implement HTML <base>
And fix/revert `xml:base` support in SimplePie https://github.com/simplepie/simplepie/commit/e49c578817aa504d8d05cd7f33857aeda9d41908

* SimplePie upstream PR merged
https://github.com/simplepie/simplepie/pull/723

app/SQL/install.sql.sqlite.php

@@ -14,6 +14,7 @@ CREATE TABLE IF NOT EXISTS `category` (
 CREATE TABLE IF NOT EXISTS `feed` (
 	`id` INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL,
 	`url` VARCHAR(511) NOT NULL,
+	`kind` SMALLINT DEFAULT 0, -- 0.20.0
 	`category` SMALLINT DEFAULT 0,
 	`name` VARCHAR(255) NOT NULL,
 	`website` VARCHAR(255),