Refactor standard compile tasks to remove Gulp

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What

Update tasks/gulp/compile-assets.js to replace the non-IE8 compile Gulp tasks with our own compile scripts. Updating the IE8 tasks depends on us removing that browser support, so has been separated into its own issue (#2713).

Currently, the scss:compile and js:compile tasks are building both app and dist files, depending on a flag. The pipelines for these files are similar, but not identical, and completely different files are built depending on the flag. We should split this work out and simplify this pipeline.

Why

We have made the decision to move away from Gulp and write our own build scripts.

Who needs to work on this

Developers

Who needs to review this

Developers

Done when

• IE8 tasks have been separated and run as is
• App and dist compilations have been split into separate tasks
• Non-IE8 compilation tasks no longer depend on Gulp
• Non-IE8 compilation tasks no longer depend on taskArguments.destination
• docs/contributing/tasks.md has been updated
• Dependencies are as up to date as is feasible (oldie is a blocker on a lot of these)
• The tasks have been moved from tasks/gulp to tasks

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I'm going to close this in favour of #2717 and I'll add some notes there about taskArguments.destination.