Browser title notification for partner join

[dmnisson]

Links

• Issue: https://github.com/UPchieve/web/issues/236

Description

• Use vue-page-visibility-awesome to track page visibility
• Display a "Volunteer has joined!" indicator in the browser title until the user shows the page

Developer self-review checklist

• [x] Potentially confusing code has been explained with comments
• [x] No warnings or errors have been introduced; all known error cases have been handled
• [x] Any appropriate documentation (within the code, README.md, docs, etc) has been updated
• [x] All edge cases have been addressed