Add anonymous posting option to topic and post creation UI

[cathyxfeiyang]

• Implement toggle for users to select whether they want to create the topic or post anonymously
• Design and implement UI changes to add this toggle to the topic and post creation page

Effort: high
Dependencies: None

Acceptance criteria:

• post and topics creation pages include a toggle for users to select anonymous posting
• manual testing for successful creation of anonymous posts and topics

[utkanuygur]

#60