Add topic dropdown to post creation

[ByteOtter]

What does this PR change?

Users are now able to choose one from a hand full of different labels for their posts when creating one. Known Issues:

• When editing a post, the label will be overwritten by the 'undefined' default selection throwing it into the Misc Channel.

Tick the applicable box:

• [x] Add new feature

• [x] UI improvement

• [x] Changed routing

• [ ] Security changes

• [ ] Testsuite
  

• [ ] General Maintenance

UI changes

provide description here or remove all unapplicable lines below

before:

after:

Added a dropdown menu in the post creation form to allow users to tag their posts with a certain topic

• [x] DONE

Links

In case your changes fix an existing issue please link it below:

Fixes: #92

• [x] DONE

Documentation

provide description about documentation done here or remove this line

• No documentation needed
  

• [x] DONE