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/form/app/utils/keep-alive.js

@@ -1,17 +0,0 @@
-import $ from 'jquery';
-import {config} from 'grav-form';
-
-const MAX_SAFE_DELAY = 2147483647;
-
-$(document).ready(() => {
-    const keepAlive = $('[data-grav-keepalive]');
-
-    if (keepAlive.length) {
-        const timeout = config.session_timeout / 1.5 * 1000;
-        setInterval(() => {
-            $.ajax({
-                url: `${config.base_url_relative}/task${config.param_sep}keep-alive`
-            });
-        }, Math.min(timeout, MAX_SAFE_DELAY));
-    }
-});