Type cast IDs from string to int

[taimoor-ahmed-1]

During the migration from forum v1 to v2, support for 2 database backends namely, mongodb and mysql, were added which meant having mongo hashes being converted to strings in Open edX and being sent to forum apis. Since, forum is going to deprecate the mongodb backend soon (work is ongoing), it won't be needing the support for mongo ids and mysql ids simultaneously. The discussion module in edx-platform and forum apis therefore, can be simplified to not type cast ids from str to int and to str again. This will clean up a lot of code in both repositories.