[ticket/12895] Rename user style setting to resolve conflict with style param

The style URL parameter, i.e. style=1, will conflict with the user's style
setting due to it also being named style. This patch with solve this conflict.

PHPBB3-12895
Merge pull request #2782 from marc1706/ticket/12895

[ticket/12895] Rename user style setting to resolve conflict with style param

* marc1706/ticket/12895:
  [ticket/12895] Rename user style setting to resolve conflict with style param