Make workflows modular

[frjo]

Hypha today has 4 workflows, all the code is in the workflow.py file. If an organisation wants to add a custom workflow they must do it in this files and maintain the changes when rebasing on new Hypha releases.

Things that could improvement this:

• Have each workflow in a separate file.
• Allow adding new workflows by adding a single file in a designated directory.
• Add a setting to activate/deactivate workflows so staff only see the needed options when creating funds and labs.