Changed input for mobile phone number to vue-tel-input in register form

Closes no issue

What does this MR do?

Changed input for mobile phone number to vue-tel-input in register form.

How confident are you it won't break things if deployed?

(be honest!)

Links to related issues

https://yunity.slack.com/archives/C74JC2V26/p1584728914081900?thread_ts=1580660156.072600&cid=C74JC2V26

How to test

Steps a reviewer can take to verify that this MR does what it says it does e.g.

1. Checkout branch locally
2. Click to "Mach mit!"
3. Click to "Jetzt registrieren!"
4. Fill field "E-Mail"
5. Fill field "Passwort"
6. Fill field "Passwortwiederholung"
7. Click to "weiter"
8. Click to a gender
9. Fill field "Vorname"
10. Fill field "Nachname"
11. Click to "weiter"
12. Fill field "birthdate" or use datepicker
13. Click "weiter"
14. Fill field "Handynummer"
15. Click to "weiter"
16. Check option "Rechtsvereinbarung"
17. Check option "Datenschutzerklärung"
18. Uncheck option "Newsletter"
19. Click to "Anmeldung absenden"
20. Check your inbox for registration mail
21. Click to "Anmelden"
22. Test the first login

Screenshots (if applicable)

Any relevant screenshots if this is a design / frontend change

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))
• Once your MR has been merged, you are responsible to update the #foodsharing-beta Slack channel about what has been changed here. They will test your work in different browsers, roles or other settings