Change all external link targets from ext to _blank

[Aquaj]

Summary of changes and context

Closes #330.

Sanity checks

• Linters pass
• Tests pass
• Related GitHub issues are linked in the description

[pil0u]

erb-lint qui casse un peu les pieds, faut ajouter une option aux liens...

            <%= f.label :accepted_coc, class: "text-sm" do
                  "I have read and I will comply with the #{link_to(
                    'Code of Conduct',
                    code_of_conduct_path,
                    target: :_blank,
                    rel: :noopener,
                    class: 'link-explicit link-internal'
                  )}".html_safe
                end %>

[Aquaj]

Erreurs de linter corrigées.