refactor: extract create logic endpoint / refactor $watchCollection

[tshuli]

Problem

• Closes [API] Extract create logic endpoint from PUT /:formId/adminform #1489
• Also refactors edit logic client controller to use $watchCollection to track changes in form_logic

Tests

Create Logic Endpoint

• Create a showFields logic. Check that logic is successfully added and build tab is successfully updated (hidden fields greyed out).
• Repeat the above with a second logic

Test using manual API calls

• Check that endpoint returns 404 when formId is incorrect

$watchCollection Refactor

• Create a form with multiple showFields logic. Delete one of them. Check that the Build tab is updated correctly (field should no longer be greyed out)
• Now change one of the logic to preventSubmit. Check that the Build tab is updated correctly (field should no longer be greyed out). Change it back to showFields. Check that the field is now greyed out on build tab

[tshuli]

@mantariksh thanks for reviewing, for re-review pls

[karrui]

lgtm with nits

[karrui]

Maybe rebase on dev for the new Github Actions :P