Although it wasn't a real security concern because we were only calling
a `find` method based on the user input, it's a good practice to avoid
using constants based on user parameters.
Since we don't use the `find` method anymore but we still need to check
the associated record exists, we're changing the `followable` validation
in the `Follow` model to do exactly that.
We were very inconsistent regarding these rules.
Personally I prefer no empty lines around blocks, clases, etc... as
recommended by the Ruby style guide [1], and they're the default values
in rubocop, so those are the settings I'm applying.
The exception is the `private` access modifier, since we were leaving
empty lines around it most of the time. That's the default rubocop rule
as well. Personally I don't have a strong preference about this one.
[1] https://rubystyle.guide/#empty-lines-around-bodies
