1.2 JS clean-up

[davwheat]

Changes proposed in this pull request:

Performs some clean-up to frontend JS, ready for 1.2.

• fixes extend.ts typing issue introduced in Convert extend util to TypeScript #2928
• fixes bad JS docblocks (contained within f476c2b)
• remove fallbacks for window.requestAnimationFrame() -- this is supported in all browsers now
• consistently return null instead of '' or [] for empty vdoms
• simplify some code and checks inside Post, PostsUserPage and slideable (40be94d)

Updating our JSDoc blocks allows us to have some form of type safety and hinting without needing to fully convert to Typescript just yet.

Reviewers should focus on:

N/A

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

[askvortsov1]

These should really all become abstract methods for v2

[askvortsov1]

Approving as long as the last few NestedStringArray are changed to Mithril.Children

[dsevillamartin]

Not tested locally (shouldn't need to) & changes are mostly docblocks which don't break anything. Nothing stands out to me so LGTM.