Added javascript to activate bootstrap tab from off-page link.

Used by post forms to link to help file for allowed bbcode.
Source of javascript: https://github.com/twbs/bootstrap/issues/2415#issuecomment-8064201
Post, edit, quickpost and signature form links to help file for allowed bbcode.

Will link if green and allowed, will not link if red and disallowed.
#787 Update help links thanks to @slightlysome
Merge branch 'slightlysome-posthelp' into version3.4