Frontend Controller Refactor

[acburdine]

closes #5192, refs #5091

This PR refactors the frontend controller, combining the functions for the homepage, tag, and author routes into one function, with the intent of using it for channels in the future.

[acburdine]

@ErisDS some of this code probably could be made better, but it works.

[ErisDS]

Really excited to see this! Such an enormous improvement to how the frontend works, test coverage for the controller automatically went up by ~10% due to the reduced duplication, and I think it's now significantly easier to read and manage 👍

The only thing I would prefer to be different is that the base URL/route for each channel be passed in as a whole rather than constructed inside the function, so it's passed in something like:

tag: renderChannel({
        name: 'tag',
        route: '/' + config.routeKeywords.tag + '/',
        ...
    }),

Or even using route: '/' + config.routeKeywords.tag + '/:slug/' and replacing the slug part. I realise the pagination part still needs to be appended depending on the case, but I think this moves us closer towards allowing users to customise the underlying routes for various things.

[acburdine]

@ErisDS fixed :)