ui-ci: let a ci-filter decide whether the tab is shown

cgit cannot know whether the ref being viewed actually has a pipeline, so
the "ci" tab is offered for every ref and only reveals a missing one once
followed. Probing the ci system from cgit is not an option: the tab is
part of the page header, so it would mean a blocking request for every
page of every repository, cgit links no http client, and job pages are
usually not readable anonymously.

Add a ci filter instead, which receives the ref, whether it is a branch or
a tag, and the expanded url, and answers with its exit status. This keeps
credentials, timeouts and caching in a script, where they belong;
filters/ci-jenkins.sh demonstrates all three against Jenkins' REST API.

The verdict is memoized, so the filter runs once per request rather than
once for the tab and again for the redirect, and it governs the page as
well as the tab, so a hidden tab cannot be reached by typing the url.

Signed-off-by: Saya Andy <saya.andy@posteo.com>

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