Add Image Loading Optimization module

[westonruter]

Summary

Add scaffolding for new Image Loading Module with some key basic functionality, namely the output buffering of template renders and the introduction of a filter for that buffered output.

Fixes #805

Relevant technical choices

When the module is active, the output buffering UI elements for Server Timing are omitted since the Image Loading Optimization module always does output buffering. Similarly, when the module is running, Server Timing will wait to send the header at the end of the new perflab_template_output_buffer filter.

Checklist

• PR has either [Focus] or Infrastructure label.
• PR has a [Type] label.
• PR has a milestone or the no milestone label.

[felixarntz]

@westonruter Can you please update this to be against a feature/* branch? I'm not sure we'd want to ship this yet just with this PR, so I think it shouldn't be against trunk until we're comfortable with the version we have.

[felixarntz]

@westonruter This looks good, just one point of feedback (plus let's change the base branch before merge to a new feature branch).

[westonruter]

plus let's change the base branch before merge to a new feature branch

@felixarntz I just now pushed up a new feature/image-loading-optimization branch off of trunk and changed the base branch of this PR to target it.

[felixarntz]

Thanks @westonruter!

[felixarntz]

Just noted that this PR and #878 are annotated with the PL 2.8.0 release, but it's not actually milestoned for a release yet since we are working in the feature branch towards a working version before moving ahead with merging into trunk. Let's use no milestone on those PRs against the feature branch.