Preload FontAwesome, JS and CSS

[davwheat]

Fixes #2741

Changes proposed in this pull request:

• Adds support for preloads in Document
• Adds preload() and preloads() methods to Frontend extender
• Adds default preloads for FontAwesome fonts
• Adds default preloads for core JS and CSS assets

The benefits of preloading FontAwesome are that these would normally only be fetched when CSS parsing is complete, which is often.

Preloading JS and CSS only makes a difference if we're preloading FontAwesome already. Browsers classify preloads as higher priority requests, meaning they would stall downloads of our JS/CSS until these are done. Adding these as preloads prevents this, and they are all fetched at once.

Overall, preloading results in a consistently faster page load time. Almost always 100-150ms faster, sometimes reaching 200ms+. If/when themes and other extensions begin preloading their assets, this could increase further.

Reviewers should focus on:

My failure to write/comprehend PHP and my blatant disregard for helpers and simplicity.

Screenshot

With preloads

Without preloads

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).

Required changes:

• Related documentation PR:

[askvortsov1]

Very excited to see this! Let's fix tests, then good to go IMO

[SychO9]

I thought that last code suggestion would for sure fix the tests :P

[davwheat]

Weirdly, it seems to work fine locally.

[davwheat]

I don't have time to work on core for the next few days, so if either of you have any ideas, please just push straight to the branch :)

[askvortsov1]

@davwheat @SychO9 issue was with a cast when preloads was called. Fixed now, and changed naming to be more descriptive of structure.

[askvortsov1]

Approving tentative on the singleton comment

[tankerkiller125]

Also going to approve tenitive the singleton comment being fixed

[askvortsov1]

LGTM