normalize labels for forms

[Robert Jäschke]

The new dropdown field "Layout" on the settings page's "settings" tab is labelled "Layout" (in English) while it should be lowercase "layout" similar to the other form labels (in general, we almost always use lowercase in the English version of BibSonomy). Please check for consistency also other labels.

[Sebastian Böttger]

message key for layout switch in settings tab added. resolves issue #2042.

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