Pluggable components from datasource plugins

- when instantiating a datasource, the datasource service checks if the
  plugin module exports Explore components, and if so, attaches them to
  the datasource
- Explore component makes all major internal pluggable from a datasource
  `exploreComponents` property
- Moved Prometheus query field to promehteus datasource and registered
  it as an exported Explore component
- Added new Start page for Explore, also exported from the datasource

public/app/plugins/datasource/prometheus/components/PromStart.tsx

@@ -0,0 +1,60 @@
+import React, { PureComponent } from 'react';
+import classNames from 'classnames';
+
+import PromCheatSheet from './PromCheatSheet';
+
+const TAB_MENU_ITEMS = [
+  {
+    text: 'Start',
+    id: 'start',
+    icon: 'fa fa-rocket',
+  },
+];
+
+export default class PromStart extends PureComponent<any, { active: string }> {
+  state = {
+    active: 'start',
+  };
+
+  onClickTab = active => {
+    this.setState({ active });
+  };
+
+  render() {
+    const { active } = this.state;
+    const customCss = '';
+
+    return (
+      <div style={{ margin: '45px 0', border: '1px solid #ddd', borderRadius: 5 }}>
+        <div className="page-header-canvas">
+          <div className="page-container">
+            <div className="page-header">
+              <nav>
+                <ul className={`gf-tabs ${customCss}`}>
+                  {TAB_MENU_ITEMS.map((tab, idx) => {
+                    const tabClasses = classNames({
+                      'gf-tabs-link': true,
+                      active: tab.id === active,
+                    });
+
+                    return (
+                      <li className="gf-tabs-item" key={tab.id}>
+                        <a className={tabClasses} onClick={() => this.onClickTab(tab.id)}>
+                          <i className={tab.icon} />
+                          {tab.text}
+                        </a>
+                      </li>
+                    );
+                  })}
+                </ul>
+              </nav>
+            </div>
+          </div>
+        </div>
+        <div className="page-container page-body">
+          {active === 'start' && <PromCheatSheet onClickQuery={this.props.onClickQuery} />}
+        </div>
+      </div>
+    );
+  }
+}