fix some edge cases in prefix-based nav expansion
fix some edge cases in prefix-based nav expansion

/r was a subset of /rust, /ruby
/java was a subset of /javascript

this fix still uses the URL to decide which nav items
to open, but instead of just checking if link path
is a prefix of the current URL,
it checks for an exact match OR if the link path appended by a '/'
is a prefix of the current URL.
This hack causes '/r/' prefix to not match /rust or /ruby,
and '/java/' prefix to not match '/javascript'

algorithmiaio/dev-center,algorithmiaio/dev-center,algorithmiaio/dev-center,algorithmiaio/dev-center