Implement announcement page

[rainasong]

Description

• Implemented the new announcement page UI
• Added API logics (both GET and POST for fetching and publishing announcements)
• Added a new card to the dashboard
• Added minor style changes for announcement modal and card components
• Added minor changes to the backend announcement.controller.ts (change return types)
• Fixed navbar does not stick to the top issue
• Temporarily solved bugs on the issues page due to updated api-schema (need to properly fix it later)

Fixes/resolves #185

Screenshots

Demo

Publish new announcements on the announcement page

https://user-images.githubusercontent.com/37087904/161267280-6ef19538-5861-4868-b491-7894023b9526.mp4

Scrolling demo (fixed navbar does not stick to the top issue)

https://user-images.githubusercontent.com/37087904/161267357-a0ac4be6-b146-4b15-b5df-8fef0e616293.mp4

Type of change

Please delete options that are not relevant.

• [x] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [x] Improvement (non-breaking change which improves existing functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

Checklist:

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I have completed these steps when making this pull request:

• [x] I have assigned my name to the issue
• [ ] I have moved the issue to the In Progress column
• [x] I have labelled the PR appropriately
• [x] I have assigned myself to the PR

Before opening the PR for review:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] I have added attributions to new dependencies and resources
• [x] I have moved the linked issue to the Review in Progress column