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/themes/grav/css/nucleus-ie10.css

@@ -1,24 +0,0 @@
-button {
-	overflow: visible;
-}
-
-input[type="checkbox"],
-input[type="radio"] {
-	box-sizing: border-box;
-	padding: 0;
-}
-
-.fixed-blocks {
-	display: block;
-}
-
-.feature-items {
-    overflow: hidden;
-}
-
-.modular .features .feature {
-	margin: 1%;
-    display: inline-block;
-    vertical-align: top;
-    float: none;
-}