Replace var in javascript with let/const

[tomhughes]

This updates the eslint config to prohibit use of var and to prefer const over let and then fixes all uses of var to respect that.

[github-actions]

	1 Warning
⚠️	Number of updated lines of code is too large to be in one PR. Perhaps it should be separated into two or more?

Generated by 🚫 Danger

[AntonKhorev]

and then fixes all uses of var to respect that

Not in unmerged pull requests.

[AntonKhorev]

If this is merged, I'll have to rewrite #5064 again. That's a bugfix PR. People are still complaining about bugs it fixes.

[hlfan]

@AntonKhorev I'd still say merge sooner rather than later just because all the arising issues can be autofixed.

[hlfan]

If you're already cleaning up after me, you can define the function as function (url, callback) for even more conciseness.

[hlfan]

Or maybe not given #5663

[nertc]

All occurrences of var are changed with let/const. Checked all changed variables and let and const are set accordingly (every variable that is not changed after declaration, is const). Also spent some time on website to test it generally, if any page threw error in the console. Everything I checked works as expected, there are no errors.

[hlfan]

All occurrences of var are changed with let/const. Checked all changed variables and let and const are set accordingly (every variable that is not changed after declaration, is const).

ESLint concurs (after 1.88 seconds)

[HolgerJeromin]

Can we merge this please?
At least I hesitate to do small things which only creates conflicts with this PR

[gravitystorm]

Needs rebase, but otherwise looks good to me. I agree it would be good to merge this as soon as we can.

[tomhughes]

I've rebased this now.

[gravitystorm]

Merged, thanks!