Add the ability to manage flags in 'bulk'

Currently, it is only possible via the UI, and via the API, to manage flags individually. It would be good to be able to update multiple flags in a single action.

Some additional thoughts on this:

1. It is possible (via the API at least) to manage multiple flags in a given change request, although the UI doesn't currently support this
2. This is likely intrinsically linked to [dependent flags](https://github.com/Flagsmith/flagsmith/issues/1674)