Initialize Theme templates in Admin

To make the [question asked on Discord](https://discordapp.com/channels/501836936584101899/506916956637495306/671455753256697858) more concrete, and make it easier to discover a solution for others, here's a replication and description.

I am trying to find a Page and render it with its template, in the event [`onAdminTaskExecute`](https://learn.getgrav.org/16/admin-panel/extending/admin-events#onAdminTaskExecute). To do this I:

1. Visit `http://localhost:8000/admin/task:processPageTemplate/admin-nonce:foobar`
2. Check that this is the wanted task and authorize it
3. [Enable Pages](https://github.com/getgrav/grav/issues/2737) in Admin
4. Initialize Themes and [Twig](https://github.com/getgrav/grav/blob/5ba4c8ee5beeed0283088b76d6aec9fe865c38fe/system/src/Grav/Common/Twig/Twig.php#L77), to [merge the Theme's templates into Twig](https://github.com/getgrav/grav/blob/5ba4c8ee5beeed0283088b76d6aec9fe865c38fe/system/src/Grav/Common/Twig/Twig.php#L97)
5. Find the `/home`-Page, included in Grav downloads by default
6. Dump `twig_paths` and get:

```php
^ array:7 [▼
0 => "C:\Users\Ole\Caddy\Task\user\plugins\flex-objects/admin/templates"
1 => "C:\Users\Ole\Caddy\Task\user\plugins\admin/themes/grav/templates"
2 => "C:\Users\Ole\Caddy\Task\user\plugins\email/templates"
3 => "C:\Users\Ole\Caddy\Task\user\plugins\form/templates"
4 => "C:\Users\Ole\Caddy\Task\user\plugins\login/templates"
5 => "C:\Users\Ole\Caddy\Task\user\plugins\error/templates"
6 => "C:/Users/Ole/Caddy/Task/system/templates"
]
```

Thus, this:

```php
$content = $this->grav['admin']->grav['twig']->processTemplate(
$Page->template() . '.' . $Page->templateFormat('html') . '.twig',
['page' => $Page]
);
dump($content);
```

Fails with `Template "default.html.twig" is not defined.`

The result is the same if I try to forcefully load the theme-templates with:

```php
foreach ($this->grav['config']->get('themes') as $theme => $config) {
$this->grav['twig']->twig_paths = array_merge(
$this->grav['admin']->grav['twig']->twig_paths,
$this->grav['locator']->findResources('themes://' . $theme . '/templates')
);
}
```

This was tested on a clean installation of Core 1.7.0-rc.3 and Admin 1.10.0-rc.3, creating a new plugin with the DevTools with this PHP:

```php
['onPluginsInitialized', 0]
];
}

public function onPluginsInitialized()
{
if ($this->isAdmin()) {
$this->enable([
'onAdminTaskExecute' => ['onAdminTaskExecute', 0]
]);
}
}

public function onAdminTaskExecute(Event $e)
{
if ($e['method'] == 'taskProcessPageTemplate') {
if (!$e['controller']->authorizeTask('processPageTemplate', ['admin.maintenance', 'admin.super'])) {
header('HTTP/1.0 403 Forbidden');
echo '403 Forbidden';
exit;
}
if (isset(Grav::instance()['admin'])) {
if (method_exists(Grav::instance()['admin'], 'enablePages')) {
Grav::instance()['admin']->enablePages();
}
}
$this->grav['admin']->grav['themes']->init();
$this->grav['admin']->grav['twig']->init();
$Page = $this->grav['admin']->grav['pages']->find('/home');

// Check registered Twig paths
dump('twig_paths');
dump($this->grav['admin']->grav['twig']->twig_paths);

// Try to forcefully load templates
foreach ($this->grav['config']->get('themes') as $theme => $config) {
$this->grav['twig']->twig_paths = array_merge(
$this->grav['admin']->grav['twig']->twig_paths,
$this->grav['locator']->findResources('themes://' . $theme . '/templates')
);
}

// Check rendered template
$content = $this->grav['admin']->grav['twig']->processTemplate(
$Page->template() . '.' . $Page->templateFormat('html') . '.twig',
['page' => $Page]
);
dump($content);
exit();
}
}
}
```

Copy of the plugin:
[task-test.zip](https://github.com/getgrav/grav-plugin-admin/files/4124445/task-test.zip)

Copy of the Grav-installation:
[grav-admin-v1.7.0-rc.3.zip](https://github.com/getgrav/grav-plugin-admin/files/4124447/grav-admin-v1.7.0-rc.3.zip)

As you write, @mahagr, Admin doesn't trigger the necessary events by default. But both `onThemeInitialized` and `onTwigTemplatePaths` are fired, and `$this->grav['admin']->grav['twig']->init()` should seemingly register the templates. Any thoughts on why they don't?