Add unit tests for bookmark routes

This PR also fixes #86

Description / Changes Made

I added unit tests for the bookmark routes.
These unit tests include testing for both bookmarking posts and recipes, adding and removing a bookmark.
Also tests for invalid requests.
Removes feed routes from the backend

None

Coverage for bookmark_routes is at 100%

Before running the test script, remember to comment out middleware in server.py. There is no real need for authentication in unit tests that deal with mock data. This can be achieved by simply commenting out @app.before_request in server.py, which is on line 45 at the time of writing this PR.
Also, make sure you have the emulator for Firebase running or this won't work.
Now, simply run run_tests.py found in the backend directory and see the coverage for bookmark_routes.

[x] I have added/updated relevant documentation, and I have followed the coding style guidelines.
[x] I have added/updated tests, and I have run the test suite and all tests pass.
[x] I have checked for any potential conflicts with other branches and fixed any merge conflicts.