announcements now come from firebase

[RakeshSeervi]

Issue Number

ISSUE #71 GOSA26

Describe the changes you've made

Used timeago package to show relative time. Added a method in firebase_storage_service.dart to get the announcements from firebase. Modified the Announcement model to create Announcement object from DocumentSnapshot.

Describe if there is any unusual behaviour of your code(Write NA if there isn't)

NA

Additional context (OPTIONAL)

Test plan (OPTIONAL)

In main.dart, set home to HomePage()

Checklist

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.
• [x] The title of my pull request is a short description of the requested changes.