We were using helper methods inside the model; we might as well include
them in the model and use them from anywhere else.
Note we're using a different logic for images and documents methods.
That's because for images the logic was defined in the helper methods,
but for documents the logic is defined in the Documentable concern. In
the past, different documentable classes allowed different content
types, while imageable classes have always allowed the same content
types.
I'm not sure which method is better; for now, I'm leaving it the way it
was (except for the fact that we're removing the helper methods).
We were converting megabytes to bytes with the `megabytes` method and
then adding a `bytes_to_megabytes` method to convert it back to bytes,
which is the same as not doing anything at all :).
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
