(core) Configure more comprehensive eslint rules for Typescript

Summary:
- Update rules to be more like we've had with tslint
- Switch tsserver plugin to eslint (tsserver makes for a much faster way to lint in editors)
- Apply suggested auto-fixes
- Fix all lint errors and warnings in core/, app/, test/

Test Plan: Some behavior may change subtly (e.g. added missing awaits), relying on existing tests to catch problems.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2785

app/client/ui2018/cssVars.ts

@@ -194,8 +194,8 @@ export const cssHideForNarrowScreen = styled('div', `
  * Attaches the global css properties to the document's root to them available in the page.
  */
 export function attachCssRootVars(productFlavor: ProductFlavor, varsOnly: boolean = false) {
-  dom.update(document.documentElement!, varsOnly ? dom.cls(cssVarsOnly.className) : dom.cls(cssRootVars));
-  document.documentElement!.classList.add(cssRoot.className);
+  dom.update(document.documentElement, varsOnly ? dom.cls(cssVarsOnly.className) : dom.cls(cssRootVars));
+  document.documentElement.classList.add(cssRoot.className);
   document.body.classList.add(cssBody.className);
   const theme = getTheme(productFlavor);
   if (theme.bodyClassName) {