Bugfix: Only show resolve post if actionable

[FelberMartin]

Problem Description

The "Resolves Post" action did wrongly show up for when selecting a users own answer post, even when the user has no tutor rights and he is not the thread base post author.

This closes #240.

Changes

• Fixed the bug
  • Changed IAnswerPost to include the parentAuthorId
  • Adapted inheriting classes
• Changed ThreadUI to make use of IStandalonePost instead of PostPojo for easier testing
• Created tests
• Restructured test file

Steps for testing

Prerequisites: user without tutor rights

• Go to a other user's post
• Write a new answer to this post
• Long press on your answer
• The "Resolves Post" action does not show anymore

[eylulnc]

Code looks good to me.
Also implementation too, I have check it for both student and instructor.
Resolve not visible for students ✅

[julian-wls]

Just tested it and the resolve post action does not show up.
Code lgtm, nice cleanup of the test file!