Update Slim to 3.12.3

[glensc]

Docs:

• Upgrade Guide: https://www.slimframework.com/docs/v3/start/upgrade.html
• Views: https://www.slimframework.com/docs/v3/features/templates.html

Fixes #278

[glensc]

Seems to me it's perhaps easier to start with a slim skeleton:

• https://github.com/slimphp/Slim-Skeleton/tree/3.1.8

and transfer logic to new locations

[glensc]

slim/views have no v3 support:

• Slim 3.0 support slimphp/Slim-Views#49

PHP Fatal error:  Class 'Slim\View' not found in vendor/slim/views/Twig.php on line 45

[glensc]

Or port to Symfony5:

• Migrate code from Silex/Sf2.8 to Symfony 5 composer/getcomposer.org#165

[glensc]

Seems slim/views replacement is slim/twig-view:

• https://www.slimframework.com/docs/v3/features/templates.html

[glensc]

The v2 code mocked POST data by overwriting $_POST:

• xhgui/tests/Controller/WatchTest.php

  Line 32 in 776b59d

  $_POST = [

This is very different in v3:

• https://glenneggleton.com/page/slim-unit-testing
• https://stackoverflow.com/questions/40867377/mock-slim-endpoint-post-requests-with-phpunit

[glensc]

Flash messages have new package:

• https://www.slimframework.com/docs/v3/features/flash.html

[glensc]

https://github.com/slimphp/Slim-Flash#using-with-twig-view

If you use Twig-View, then slim-twig-flash may be a useful integration package:

• https://github.com/kanellov/slim-twig-flash

[glensc]

Revert unintended close:

• Add release automation using laminas/automatic-release #415 (comment)