[Users] Folder option open selected message on folder open

[Olivier Brunel]

Hey,

Here's a small patch to add a folder option open selected message on
folder open, which can of course be "Default" to follow general
preferences (the default), but allows to override to Yes or No on a
per-folder basis.

(Something I find useful in a few occasions, e.g. a spam folder where
you don't need/want the selected message to be opened, esp. if it's
gonna take a few seconds because it contains hundreds of links or
whatever crap no one cares about.)

The two small patches before that are just small bits :

- first removing some useless code to make things clearer;
- and some minor refactoring for similar Default/Yes/No folder options,
  so they all simply use the same OptionType instead of having to
  re-define the same thing over & over again for no real benefits.

Cheers,
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