Make one image URL per source on the generator page for host case and an empty path (closes #293)

The generator page put the source host and path into the encrypted URL
as entered, so CDN.Example.com and cdn.example.com, or a source URL with
no path and with the path /, made two cache keys and two upstream
requests for one image. It now parses the source URL with the host
lower-cased and an empty path written as /, as /v1/image/ reads a
source URL, before the check that pixa fetches the URL as entered. That
is a small function of its own because HandleGenerateURL is at the
linter's length limit. The README says what the token keeps.

Model: opus-5-5

README.md

@@ -473,9 +473,11 @@ nor change what it asks for.
    `jxl` for a form with no format.
 
 The token holds the source's host, path and query and the size, format, quality,
-fit and expiry, encrypted with a key derived from `signing_key`. The source
-URL's scheme is not kept: the image is fetched like any other (see Routes), and
-the blocked networks still apply.
+fit and expiry, encrypted with a key derived from `signing_key`. The host is
+kept in lower case and an empty path as `/`, as for `/v1/image/`, so
+`https://CDN.Example.com` and `https://cdn.example.com/` are one source image,
+fetched and cached once. The source URL's scheme is not kept: the image is
+fetched like any other (see Routes), and the blocked networks still apply.
 
 How long the URL lasts is chosen on the page, from 1 minute to 1 year, or never.
 The expiry is fixed in the token when the URL is made and cannot be changed or