i18n: refs to lang.de.yml - Wall.vue, ThreadPostActions.vue, ThreadForm.vue, Thread.vue and RequestForm.vue

Relates #824 (closed)

What does this MR do?

Redirect the refs from Wall.vue, RegisterMobilePhone.vue, ThreadPostActions.vue, ThreadForm.vue, Thread.vue and RequestForm.vue to lang.de.yml

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

Mostly

Links to related issues

#824 (closed)

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. Login
3. Check ThreadPostAction.vue changes (See Screenshot)

• Go to a thread
• Hover the mouse about the "+"-Button and see the text (should be: Eine Reaktion hinzufügen)

4. Check ThreadForm.vue (See Screenshot)

• Go to a thread
• Scroll down to the end of the thread.
• In the box in the bottom of the site the header should be "Antworten..." and the Send-Button should be named "Senden"

Wall.vue not possible to check, because it is not used at the moment. Thread.vue not possible to check, because the text is not used. RequestForm.vue not possible to test because the text is only visible in error messages, if some problems occurs.

Screenshots (if applicable)

3. 
4. 

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