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/
So now we've got a component receiving records (goals or targets) and a
related model (Debate, Proposal, ...), with optionally a link to see
more tags.
This way we simplify some logic since the `TagList` classes were dealing
with too many cases (a record is passed, a class name is passed, a limit
is passed), ... Now `TagList` only deal with the natural `TagList` case,
which is listing the tags for a record. The case where a class name is
passed is used in the `TagCloud` class.
This way we'll be able to add targets as well. Besides, having two
classes in the CSS selector will allows to overwrite styles such as
`.debate-show ul li`.
On the other hand, we need to add a stylesheet for the filters as well.
