Since targets didn't have a title but only a long description, every
form allowing to select targets was pretty much unusable: we either
displayed just the code or the whole description.
Now, with a concise title, it's easier to find and select the desired
target.
The titles have been copied from The Global Goals page [1].
Note we're using the `short_title` I18n key for the `title` method and
the `long_title` I18n key for the `long_title` method. We can't use
`title` as I18n key instead of `short_title` because it would affect
existing translations.
[1] https://www.globalgoals.org/
Similar to what we do with settings, only for settings we return the
value of the setting (which is what we're going to need most of the
time), and here we return the object.
Since data for this model (title and description) is not generated in
CONSUL but by the United Nations, we aren't storing it in the database
but in our YAML translation files.
The reasoning is as follows. Suppose that, a few months after CONSUL
gets SDG support, a new language is added to CONSUL.
With YAML files, getting the texts in the new language would mean
updating CONSUL to include the new language.
But if we store these texts in the database, it means we have to update
the databases of all existing CONSUL installations, either each
installation by themselves (duplicating efforts) or running a rake task
(which we would have to write each time).
So we believe using translations works better in this case.
We're still storing records in the database with the code, so they can
be easily referenced via `has_many` or `has_many :through` associations.
