Prepend REACT_SDK_VERSION with a v to match riot-web version output
Add simple helper to construct version/commit hash urls
var -> let/const and prepend olmVersionString with v for same reason
for both matrix-react-sdk and riot-web, if unknown/local don't do anything
else try to create a link to the commit hash/tag name
Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>