Add /links and /backlinks endpoints

Both tiddlers and revisions get links. Only tiddlers get backlinks.

Format is JSON only and index only
Extract tiddler/space information from linked urls

info can be extracted from both standard links and shortlinks (via
regexp) where the links either starts with a "/" or with the root_url
(as defined in config/environments/*.rb).
Improve the link serializer
Add space name to backlinks in html view
Update tests for new transclusion syntax
Add inline link recognition

This is only partial recognition of inline links (i.e. it won't
recognise query parameters, handle matching brackets, etc like redcarpet
will) as the only thing we actually need it for is to extract link
information about any spaces/tiddlers being linked to (for e.g. backlink
purposes) so we don't care about much of the url anyway.