Formalize Sentry console logging practices

[rk-for-zulip]

At present, in contrast to its pre-1.x.x behavior, Sentry does not send console events as breadcrumbs. (It may also not send console.error as an error event; this needs to be confirmed.) This was done in this commit.

There is an issue report open here to restore this behavior. This is probably what most people will usually want, but it's also probably not what we want: as noted in #3733, there are a number of places where we upload data to Sentry in a more processed form than can be provided to the console, but where we still want that data to be flattened and printed to the console as a potential debugging aid.

We should explicitly specify this behavior, rather than relying on the current default – e.g., by adding the following in our Sentry.init call:

// This overrides the default integrations, rather than replacing them.
// See: https://docs.sentry.io/platforms/javascript/#sdk-integrations
integrations: [
  new Sentry.Integrations.Breadcrumbs({
    console: false,
  }),
],

(Beware! The above has not been tested at all. The necessity and tedium of so doing is why this is an issue rather than a PR.)