fix: rule require-i18n-strings reports mailto false positive

In GitLab, there are a few places in Vue where a mailto link (mailto:email@host.com) is constructed:

• app/assets/javascripts/admin/users/components/user_avatar.vue
• app/assets/javascripts/diffs/components/commit_item.vue
• app/assets/javascripts/issuable/components/issuable_by_email.vue

All of these get flagged as failing @gitlab/require-i18n-strings. Therefore, this MR attempts to fix these false-positives by adding a check that the string doesn’t start with mailto:.

I’d say it’s highly unlikely that an externalized string should start with mailto:. 🤔