CLI documentation

This PR was originally meant to document the CLI changes of openmicroscopy/openmicroscopy#498 and openmicroscopy/openmicroscopy#514. Since the Command Line Interface documentation had been directly imported from trac, I ended up significantly rewriting the CLI documentation with the help of @ximenesuk.

Summary of main changes:
- CLI documentation is now split into three pages (users, sysadmins, developers)
- User documentation currently describes `sessions` & `import` plugins
- Sysadmin documentation currently describes `config`, `db`, `admin`, `user` & `group` plugins
- A new role `:omerocmd:` is defined in conf.py which turns CLI commands into bold face and create an entry in the generated index.

Note: this PR documents changes occuring in the `dev_4_4` branch post the `v.4.4.5` tag. Thus it should only be be released together with the code release and should not be merged until a full release is scheduled.