* Refactor: Use generic to change func StringInSlice to Contains
* Refactor: Use generic to change func stringNotInSlice to Contains
Make utils.go (dir server/public/utils) and func Contains
* Refactor: Move func Contains from channels/utils to public/utils
Move func Contains from channels/utils to public/utils
Fix import declarations line
* Docs: Add a description of the Contains function
* Test: add TestContains
Add a test code for a Contain function