Meta: Re-implement CiviCRM Standalone (CMS-less CiviCRM)

Follow-up to #1153 (closed), and more context in this wiki page, this is a meta-issue to track the re-implementation of CiviCRM Standalone.

• Base classes (Hook, System, Permission and css/tpl so that Standalone can boot / proof-of-concept
• Basic composer project to handle the code-case (see: https://github.com/civicrm/civicrm-standalone)
• Composer template project that follows best-practices
• Implement a breadcrumb (was provided by the CMS)
• Login page
• User management and permissions
• Password reset by email
• Third-party authentication for SSO, such HybridAuth or LDAP (via extension?)
• Clean URLs
• Language switcher
• GenCode
• Installer (using 'civicrm-setup')
• Buildkit (mostly, requires cv support, see: https://github.com/civicrm/civicrm-buildkit/pull/662)
• cv (workaround works: CIVICRM_SETTINGS="/var/www/standalone/data/civicrm.settings.php" cv sql:cli)
• Backend themes (test known themes)
• Front-end theme
• Releaser support (create tar.gz for official downloads)
• Demo site
• Installation documentation
• Getting started dashlet - alert.civicrm.org needs to be updated to understand "uf=Standalone" since it currently gives error 500, e.g. https://alert.civicrm.org/welcome?prot=1&ver=5.46.alpha1&uf=Standalone&sid=blah&lang=en_US&co=
  • infra/community-messages!2 (merged)
• Without a high-level error handler that the cms (at least drupal) usually provides, wacky errors end up with unpredictable results and no nice error page and no easily-accessible logging. Sometimes you can get output sent after headers which isn't the actual error and isn't helpful.
• How to treat cms.root. The rest of the work so far is treating "standalone" as if it were a cms, so it should probably return something for it.

Known minor bugs:

• On the /civicrm/admin page, the links are missing the leading /
• Because CRM/Utils/System/Standalone.php automatically sets userID = 1, that CID is the default org record, so some public forms such as Event Registrations, behave incorrectly. Will be fixed when we implement users.
• Might be a local issue but I [DaveD] get the old files system status warning. There's an open ticket somewhere about this happening for drupal 8 sometimes so might be similar.

PRs:

• Base Standalone classes: https://github.com/civicrm/civicrm-core/pull/22227 - needs review
  • Some drupal references still in Standalone.php, e.g. the contact entityref New Individual popup triggers one.
  • $docTitle in standalone.tpl might not be the right choice. It's not set every page so gives warnings. Maybe it should be set every page in core.