(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/server/lib/gristSessions.ts

@@ -61,7 +61,7 @@ function createSessionStoreFactory(sessionsDB: string): () => SessionStore {
           // Doesn't actually close, just unrefs stream so node becomes close-able.
           store.client.unref();
         }});
-    }
+    };
   } else {
     const SQLiteStore = require('@gristlabs/connect-sqlite3')(session);
     promisifyAll(SQLiteStore.prototype);
@@ -72,7 +72,7 @@ function createSessionStoreFactory(sessionsDB: string): () => SessionStore {
         table: 'sessions'
       });
       return assignIn(store, { async close() {}});
-    }
+    };
   }
 }