Adding posts search functionality - back end

[njouud]

This is a sub-issue of issue #20, only concerning the back end code of implementing the posts search function without test code.

Estimated effort:
The task required 5 days to complete, with 2 days of debugging.

Dependencies among other tasks:
There are no dependencies among other tasks.

Initial assignments for team members:
Initially assigned to Najoud Al-Talib: @njouud

Milestones to indicate which sprint the issue should be completed in:
Sprint 1

Acceptance criteria:
This issue is completed once the the posts search function is implemented in the same format as the user search, keeping in mind the differences between the models.