Add debug logs to troubleshoot the creation of the Indiekit/Express server and the connection to MongoDB

[jackdbd]

I think it's useful to understand what's happening in the bootstrap method of the Indiekit class, and in general when Indiekit creates the Express server and connects to a MongoDB database. A few debug logs placed in key phases of the server creation and database connection phases can help a long way. For example, some time ago I spent some time trying to figure out why Indiekit would not connect to MongoDB.

The debug logs of this PR can be selectively enabled by setting the DEBUG environment variable:

DEBUG=indiekit:index,indiekit:express,indiekit:mongodb

[paulrobertlloyd]

Few tweaks, but this looks very helpful for debugging. Want to make sure not overly noisy, are all these messages useful? (I think there’s a way to set log level for a given message, maybe at some point we should do that?)

[paulrobertlloyd]

Had to create a this PR in #758 so that I could merge this change. Thanks @jackdbd!