Uphold consistent use of quotation marks

This is a stylistic task, and it's one that a linter will probably catch, but in case that isn't set up or it doesn't, one thing we should be consistent with stylistically is the quotation marks. I believe in JS, single-quotes are preferable to double-quotes.

For example, some lines:

metriq-api/service/userService.js

Line 4 in 409f96e

const MongooseService = require( "./mongooseService" );

metriq-api/service/userService.js

Line 6 in 409f96e

const UserModel = require( "../model/userModel" );

metriq-api/service/userService.js

Line 10 in 409f96e

const e = require("express");

contain double-quotes, where they should instead probably prefer single-quotes.