ui-atom: generate valid Atom feeds

ui-atom: generate valid Atom feeds

Fixes several RFC 4287 violations:

> 4.1.1. The "atom:feed" Element
>    o  atom:feed elements MUST contain exactly one atom:id element.
>    o  atom:feed elements SHOULD contain one atom:link element with a rel
>       attribute value of "self".  This is the preferred URI for
>       retrieving Atom Feed Documents representing this Atom feed.
>    o  atom:feed elements MUST contain exactly one atom:updated element.

An atom:id element is generated from cgit_currentfullurl(), and an
atom:link element with a rel attribute of "self" is generated with
the same URL. An atom:updated element is generated from the date
of the first commit in the revision walk.

> 4.1.2.  The "atom:entry" Element
>    o  atom:entry elements MUST NOT contain more than one atom:content
>       element.

The second atom:content element with the type of "xhtml" is removed.

> 4.2.6.  The "atom:id" Element
>    Its content MUST be an IRI, as defined by [RFC3987].  Note that the
>    definition of "IRI" excludes relative references.  Though the IRI
>    might use a dereferencable scheme, Atom Processors MUST NOT assume it
>    can be dereferenced.

The atom:id elements for commits now use URNs in the "sha1" or
"sha256" namespaces. Although these are not registered URN namespaces,
they see use in the wild, for instance as part of magnet URIs.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>