Restructure severity help function to match status

The code structure was vastly different in severity.py to status.py, so
I mostly copied the structure from there and adjusted it to suit
severity.

The structure in status.py looked (to me) cleaner, more organised, and
easier to work with.

Also, users are now referred by "be severity --help" to "be set --help",
in a manner similar to "be status --help".
For those that don't know that severity can be adjusted on a per
repository basis, this seems extremely helpful. A similar message
appears for status, but not here.

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