[en] Uniformize Capitalization Rules

Capitalization in the en-gb language pack is slightly inconsistent:

"Jump to"
"New Topic"
"Subscribe topic"
"Return to Board Index"
"Board index"

There's obviously some dilemma between using title case and sentence case. How about settling down a general capitalization rule?

As far as I know, the issue applies to the "en" and "en-us" language packs and not to the "fr" pack, which uses sentence case everywhere as suggested by the French linguistic rules.