While running the `dev_seed` twice, as we do in the tests, we were
getting the following warnings:
```
db/dev_seeds/proposals.rb:1: warning: already initialized constant
IMAGE_FILES
db/dev_seeds/budgets.rb:1: warning: already initialized constant
INVESTMENT_IMAGE_FILES
```
So we're extracting a method which allows us to use local variables
while removing duplication.
We had this warning with every version of Ruby, not just Ruby 2.7, but
since we're getting rid of all the warnings, we're taking care of this
one as well.
e19205cfe5