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 e49c578817

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

app/SQL/install.sql.pgsql.php

@@ -13,6 +13,7 @@ CREATE TABLE IF NOT EXISTS `_category` (
 CREATE TABLE IF NOT EXISTS `_feed` (
 	"id" SERIAL PRIMARY KEY,
 	"url" VARCHAR(511) UNIQUE NOT NULL,
+	"kind" SMALLINT DEFAULT 0, -- 0.20.0
 	"category" SMALLINT DEFAULT 0,
 	"name" VARCHAR(255) NOT NULL,
 	"website" VARCHAR(255),