Use `const` whenever possible

[elisee]

We've been using let for declaring variables almost everywhere. This is a good thing, as it is better-behaved than var (yay for block-scoping!).

But ES6 / TypeScript actually comes with const keyword. As a general rule, we should use it whenever possible, that is, whenever the variable identifier isn't meant to be reassigned.

Rationale: It will make variable identifiers that are meant to be reassigned easier to locate, and will reduce the likelihood of accidental reassignements.

The const declaration creates a read-only reference to a value. It does not mean the value it holds is immutable, just that the variable identifier cannot be reassigned.

Doing this in the whole codebase all at once might be a pain, so feel free to submit a pull request that just takes care of a file or two. Over time, we'll get there.

[abclive]

I will have a look at it :)