Composer compatibility and performance improvements

[PitaJ]

@julian @baris @psychobunny

Last night, took a look at the composer and improved the performance substantially. I also changed the positioning to use top instead of translate for better compatibility.

NodeBB/nodebb-plugin-composer-default#12

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Also, just want to remind you of my other PRs, NodeBB/nodebb-theme-persona#171 and NodeBB/NodeBB#3769

[julian]

Thank you for the reminders Helps us keep on top of things!

[exodo]

good job

[PitaJ]

More changes here, there's now NodeBB/NodeBB#3807, which replaces utils.findBootstrapEnvironment to make it much faster. Also made another branch with-css which uses CSS for the .write-preview-container positioning.