Add ability to manually override templates in PageIndex

[live627]

This will allow us to remove "Pages" from inline topic listings in the messageinex

[Sesquipedalian]

Suggested changes:

1. $templateOverrides → $template_overrides, merely for consistency with style used elsewhere (i.e. underscores for variables, camel case for methods).
2. Call $this->setTemplateOverrides() from constructor rather than duplicating code.
3. For all cases, set the default value of $template_overrides parameter to [] and set the allowed type to array rather than ?array. Passing an empty array to $this->setTemplateOverrides() is harmless, so simpler and more consistent is better.