unify redirects, document mapping and dynamic urls

[amitu]

-- fastn.url-mappings:

;; static url mapping
/fastn-for-everyone/ -> university/fastn-for-everyone/index.ftd
;; dynamic url mapping
presentation-page: /p/<string:team-slug>/<string:presentation-slug>/ -> presentation.ftd
;; redirects:
/foo/ -> /bar/
/g/ -> https://google.com

When linking to presentation page, we need to construct ftd.route or-type dynamically:

-- or-type route:

-- record presentation-page:
string team-slug:
string presentation-slug:

-- end: route

Eg:

-- ftd.text: hello
-- ftd.text.route.presentation-page:
team-slug: fifthtry
presentation-slug: my-ppt