(core) Add rules to eslint to better match our coding conventions.

Summary:
We used tslint earlier, and on switching to eslint, some rules were not
transfered. This moves more rules over, for consistent conventions or helpful
warnings.

- Name private members with a leading underscore.
- Prefer interface over a type alias.
- Use consistent spacing around ':' in type annotations.
- Use consistent spacing around braces of code blocks.
- Use semicolons consistently at the ends of statements.
- Use braces around even one-liner blocks, like conditionals and loops.
- Warn about shadowed variables.

Test Plan: Fixed all new warnings. Should be no behavior changes in code.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2831

app/gen-server/ApiServer.ts

@@ -364,7 +364,7 @@ export class ApiServer {
       const userId = getAuthorizedUserId(req);
       await this._dbManager.connection.transaction(async manager => {
         const user = await manager.findOne(User, userId);
-        if (!user) {return handleDeletedUser(); }
+        if (!user) { return handleDeletedUser(); }
         user.apiKey = null;
         await manager.save(User, user);
       });