Make discovery match the Tent spec

[longears]

The code works right now on tent.is but it's not exactly following the Tent spec.

The proper thing to do is:

• First of all, all of these requests need to use this header:

Accept: application/vnd.tent.v0+json

• Start with the entity URL, https://longears.tent.is
• Do a HEAD request to get the HTTP Link headers which will point to the profile API address: https://longears.tent.is/tent/profile. If there are no Link headers, do a GET request and look in the HTML body for <link> tags. These URLs can be relative or absolute.
• Use the profile API URLs to get the profile by doing a GET request. This is the GET /profile API method.
• In the returned JSON, look up json['https://tent.io/types/info/core/v0.1.0']['servers'] which will be a list of API endpoints. In the case of tent.is, there will be only one and it will be https://longears.tent.is/tent unless the user changed it by updating their profile using the API
• Also in the returned JSON, look up json['https://tent.io/types/info/core/v0.1.0']['entity'] and use that as the entity URL from now on. It might be different than the one you started with.

Notes on Link headers: python's requests library only gives us the last one when more than one is specified. We can fix that by parsing the Link header ourselves by grabbing it from r.headers['Link'] and splitting it into separate items. This would happen in TentApp.discoverAPIUrls().

See https://tent.io/types/info/core/v0.1.0 for details of the HTTP Link header
See https://tent.io/docs/app-server for an example JSON response to a GET /profile
See tent/tent.io#72 for more discussion about this

Also, detection of tags in the HTML works but is not robust. We should use an HTML-parsing library to find the tags.

[longears]

working on this now...

[longears]

Done! Needs testing on a server that uses relative URLs in its Link header.

[followben]

Thanks for this David - your description helped enormously with followben/TPTentClient@082139c