Bug 944749 - [Settings][l12y] Use different labels for menu items and headers

Don't reuse labels in different contexts (and with different space available).
Use fallback to original string for headers ({{id}}) to reduce impact on existing localizations.
Merge pull request #14224 from flodolo/separate_header

Bug 944749 - [Settings][l12y] Use different labels for menu items and headers, r=kaze