Remove v-html from app/assets/javascripts/boards/components/modal/empty_state.vue

As part of the effort to remove v-html from the codebase, we need to replace it with a safer alternative.

See parent epic for more details and examples.

Implementation Plan

• Replace v-html with v-safe-html by using GlSafeHTMLDirective
• Is the value being passed to v-html sanitized before in frontend?
  • Remove that part, and
  • Replace every other v-html that uses that value with v-safe-html
• Remove the comment /* eslint-disable vue/no-v-html */
• Follow standard code review process