Add variable %{latest_tag} available for badge link and url
Could you please add a variable with the last tag (eg : `%{latest_tag}`) available for badges link and url.
## Implementation Guide
1. Update the `app/models/badge.rb` to include `last_tag` in the `PLACEHOLDERS` hash, e.g.:
```ruby
'last_tag' => -> (project) { TagsFinder.new(project.repository, PARAMS).execute }
```
2. Update the ~documentation, as `doc/user/project/badges.md` and `doc/api/project_badges.md`
3. Update the `app/assets/javascripts/badges/components/badge_settings.vue` to address new variable.
4. Add missing tests, verify that badge has been displayed correctly, etc.
issue