Feature/new archiving (#2335)

* Change archiving config page layout

I've changed some wording and moved actions into a
maintenance section.

* Update purge action

Now we have more control on the purge action. The configuration allows
us to choose what to keep and what to discard in a more precise way.
At the moment, the configuration applies for all feeds.

* Add purge configuration on feed level

Now the extend purge configuration is available on feed level.
It is stored as attributes and will be used in the purge action.

* Update purge action

Now the purge action uses the feed configuration if it exists and
defaults on user configuration if not.

* Add empty option in period list

* Fix configuration warnings

* Add archiving configuration on categories

See #2369

* Add user info back

* Add explanations in UI

* Fixes for SQLite + error + misc.

* Fix invalid feed reference

* Short array syntax

Only for new code, so far

* Fix prefix error

* Query performance, default values

Work in progress

* Fix default values and confirm before leaving

Form cancel and confirm changes before leaving were broken.
And start taking advantage of the short echo syntax `<?= ?>` as we have
moved to PHP 5.4+

* More work

* Tuning SQL

* Fix MariaDB + performance issue

* SQL performance

* Fix SQLite bug

* Fix some attributes JSON encoding bugs

Especially for SQLite export/import

* More uniform, fix bugs

More uniform between global, category, feed settings

* Drop special cases for old articles during refresh

Instead will use lastSeen date with the new archiving logic.
This was generating problems anyway
https://github.com/FreshRSS/FreshRSS/issues/2154

* Draft drop index keep_history

Not needed anymore

* MySQL typo

Now properly tested with MySQL, PostgreSQL, SQLite

* More work for legacy values

Important to avoid overriding user's preference and risking deleting
data erroneously

* Fix PHP 7.3 / 7.4 warnings

@aledeg "Trying to use values of type null, bool, int, float or resource
as an array (such as $null["key"]) will now generate a notice. "
https://php.net/migration74.incompatible

* Reintroduce min articles and take care of legacy parameters

* A few changes forgotten

* Draft of migration + DROP of feed.keep_history

* Fix several errors

And give up using const for SQL to allow multiple database types (and we
cannot redefine a const)

* Add keep_min to categories + factorise archiving logic

* Legacy fix

* Fix bug yield from

* Minor: Use JSON_UNESCAPED_SLASHE for attributes

And make more uniform

* Fix sign and missing variable

* Fine tune the logic

phpcs.xml

@@ -3,8 +3,6 @@
 	<description>Created with the PHP Coding Standard Generator. https://edorian.github.com/php-coding-standard-generator/</description>
 	<!-- to circumvent https://github.com/squizlabs/PHP_CodeSniffer/pull/1404 -->
 	<arg name="tab-width" value="40"/>
-	<exclude-pattern>./static</exclude-pattern>
-	<exclude-pattern>./vendor</exclude-pattern>
 	<exclude-pattern>./lib/SimplePie/</exclude-pattern>
 	<exclude-pattern>./lib/PHPMailer/</exclude-pattern>
 	<exclude-pattern>./lib/http-conditional.php</exclude-pattern>
@@ -28,9 +26,6 @@
 		<exclude-pattern>./app/install.php</exclude-pattern>
 		<!-- @todo remove test exclusion -->
 		<exclude-pattern>./tests/app/</exclude-pattern>
-		<!-- @todo remove SQL exclusion -->
-		<exclude-pattern>./app/SQL/install.sql.mysql.php</exclude-pattern>
-		<exclude-pattern>./app/SQL/install.sql.pgsql.php</exclude-pattern>
 		<properties>
 			<property name="lineLimit" value="100"/>
 			<!-- needs to be large to accomodate extra large tab width to circumvent https://github.com/squizlabs/PHP_CodeSniffer/pull/1404 -->