Move plugins to manifest, pin Docker version, add Makefile

- Add plugins.txt listing all plugins for reproducible installs
- Add Makefile with setup/start/stop/install-plugins targets
- Remove user/plugins/ from git tracking
- Pin Docker image to 1.7.49.5-ls244

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

plugins/admin-media-replace/CHANGELOG.md

@@ -1,32 +0,0 @@
-# v1.0.4
-## 12/11/2018
-
-1. [](#feature)
-    * Update documentation
-
-# v1.0.3
-## 2/24/2018
-
-1. [](#new)
-    * Check that dependencies are enabled before loading
-
-# v1.0.2
-## 2/24/2018
-
-1. [](#new)
-    * Check for dependencies before loading
-
-# v1.0.1
-##  1/25/2018
-
-1. [](#new)
-    * Add dialog
-    * Add quicksend option
-    * Enforce extension, image, and file-rename options
-    * Refactor project layout
-    
-# v1.0.0
-##  1/24/2018
-
-1. [](#initial)
-    *