Prologue: I know about gems.
I use Drupal at work. In case you don't know, Drupal is modules. There is a framework and a core framework, but when you build a site in Drupal, installing and configuring modules is most of the work.
So what, being new to Rails, I have a pet project and I am trying to build a login/user system. I've read the question about it here on Stack Overflow.
The procedure is
If you make 20 websites in a year and 18 have the same login system, what can you do to minimize step 2?
Is there community support or activity I'm not aware of around this? Is this configuration over convention?
Rails has "plugins", which let you modularize pretty well. In fact, when you use a gem with your app, you often have a stub plugin to load the code from the gem. Take a look at the book!
Then, take the whole thing a step further to use Rails Engines, which let you plug in whole app chunks without a whole lot of extra overhead.
Note that the standard solutions for authentication are, in fact, plugins.