Refactoring

[np]

• Move finally into the new module Control.Monad.Except.Missing
• Move formH into the new module Servant.Missing
• Add a MonadAction context synonym to be used instead of some occurrences of Action
• Use for_ instead of mapM_ when this reads better
• Use $> instead of >> and return
• Use .= instead of modify and .~
• Use %= instead of modify and %~
• Use use instead of gets and ^.
• Use preuse instead of gets and a manual version of ^?
• Factor the tab switching as switchTab and switchTab'

[fisx]

This is a considerable improvement, thanks! I fixed a typo and eliminated a FIXME. If you have no objections you can merge.