Can't load scripts or inline styles in HTML Galley

[NateWr]

Describe the bug
You can use a special "context" for styles that you want to be included in the HTML Galley. Some these use this to apply styles to all HTML galleys.

class ExampleTheme extends ThemePlugin
{
    public function init()
        $this->addStyle(
            'example', 
            'css/galley.css', 
            ['contexts' => 'htmlGalley']
        );
    }
}

However, the inline option, which prints styles inline in a <styles> tag, is not working. This can be used to set CSS variables based on theme options (like colors).

Also, the htmlGalley context is ignored when using ThemePlugin::addScript() or TemplateManager::addJavaScript().

To Reproduce
Steps to reproduce the behavior:

1. Add the code in Additional Information below to DefaultThemePlugin::init().
2. Upload a HTML galley.
3. View the HTML galley
4. See that the styles and script are not loaded in the <iframe>.

What application are you using?
stable-3_3_0

Additional information
This is sample code you can use to reproduce this bug. Add it to the top of DefaultThemePlugin::init():

$this->addStyle(
	'inline-style', 
	'body { --example-var: 16px; }',
	[
		'contexts' => 'htmlGalley',
		'inline' => true,
	]
);
$this->addScript(
	'script', 
	'any-script.js',
	[
		'contexts' => 'htmlGalley',
	]
);

[NateWr]

The following PRs inject scripts and inline styles into the HTML of a HTML galley.

PRs:
#10676

Tests:
pkp/ojs#4548

[asmecher]

Thanks, @NateWr! @jardakotesovec, would you mind reviewing the PR above?