rework "check if page exists"

[posativ]

Currently, Isso uses the browser-provided "Origin" that matches its internal host list (or the first as fallback if no match) to send a GET request to the requested page. If the request succeeds, comments can be posted to that page.

However this has several drawbacks: if – for some reasons – Isso can not connect to the internet, new comments always fail with a "Not Found" response. Secondly, Isso is not limited to a sub url e.g /foo, it happily takes comments for /bar as well.

Roadmap:

• [ ] default thread provider: fetch URL if not in database, extract title or fallback to isso-title attribute. Use browser origin or try a custom host.
• [ ] new thread provider: map relative URI to filesystem (e.g. to static blog)
• [ ] also introduce a dummy provider that happily accept comments on everything without any network activity
• [ ] write functional tests

[Hywan]

:+1: