Support for custom templates

[nikosdion]

Some white-labelling use cases necessitate more than just customising the CSS. People need to use a custom template, e.g. using a fork of the default template and do view template overrides.

For example, changing the logo of the application requires the following:

• A custom template with index.php changed in three places: favicon, mask, light mode logo, dark mode logo.
• View template override for the Login/default.blade.php file

Right now, the template used is hard-coded, but it does not have to be.

We need to make it possible for people to select a template other than the default and document how to create their own template.