Add output buffering checkbox to Server-Timing screen

[westonruter]

Summary

Implements output buffering checkbox as discussed at #784 (comment).

Relevant technical choices

A new boolean output_buffering array item is added to the perflab_server_timing_settings option. This value, defaulting to false, will be used if there is no perflab_server_timing_use_output_buffer filter present.

Screenshots

When the option is not enabled, the note at the top of the screen remains:

When the option is enabled, the note at the top is removed:

When a perflab_server_timing_use_output_buffer filter is added which returns true, the checkbox is checked and disabled with a note about the filter being present:

The same is true when the filter returns false:

Checklist

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

[felixarntz]

Thanks @westonruter, this looks great so far. I left some feedback, most importantly I'm thinking we could display the field in a slightly more prominent way.

[felixarntz]

@westonruter Given that this applies to the newly added screen, I've added this to the 2.6.0 milestone and updated the base branch accordingly, let's try to merge it in time.

[felixarntz]

@westonruter Some mostly very minor follow-up feedback.

[felixarntz]

This is almost there. A few last points of feedback.

[felixarntz]

@westonruter One final nit-pick on wording, but otherwise this looks great! 🎉

[mukeshpanchal27]

Thanks @westonruter for the PR. LGTM! I left some non blocking feedback.

[felixarntz]

@westonruter Could you take a look at @mukeshpanchal27's feedback please so that we can try to merge this today?

[felixarntz]

@westonruter @mukeshpanchal27 In light of the CSS feedback, after taking a closer look I've pushed an update in 7e8c9c5 that changes the markup to match what is used on WP core's own settings screens, specifically for the uploads_use_yearmonth_folders UI control (which for example still uses a .form-table, which was previously missing here and therefore required several style tweaks to look consistent with other core settings UI). This change also means that almost all of the custom CSS was removed while maintaining a consistent appearance.