In all=no conditionals, depend on the influences, not the test pagespec

Previously, if a page like `plugins/trail` contained a conditional like

    [[!if test="backlink(plugins/goodstuff)" all=no]]

(which it gets via `templates/gitbranch`), then the
[[plugins/conditional]] plugin would give `plugins/trail` a dependency on
`(backlink(plugins/goodstuff)) and plugins/trail`. This dependency is
useless: that pagespec can never match any page other than
`plugins/trail`, but if `plugins/trail` has been modified or deleted,
then it's going to be rendered or deleted *anyway*, so there's no point
in spending time evaluating match_backlink for it.

Conversely, the influences from the result were not taken into account,
so `plugins/trail` did not have the
`{ "plugins/goodstuff" => $DEPEND_LINKS }` dependency that it should.

Invert that, depending on the influences but not on the test.

Bug: http://ikiwiki.info/bugs/editing_gitbranch_template_is_really_slow/
partially revert 154c4ea9e

explicitly en- and decoding xmlrpc requests in the python proxy broke
plugins on debian sid, while it was introduced to fix breakage in debian
stable. it is assumed that an xml module involved changed its behavior
from str to unicode, this patch handles both cases.
Added svg figure
inline: postform=no should take precedence over rootpage existing

If someone has explicitly disabled the postform, it seems reasonable
from a least-astonishment point of view for that to take precedence
over rootpage, even though that makes rootpage useless.

Also add a regression test; so far, this is all it tests.
comments: use comments_pagespec for authorization, not just UI
trail: don't generate a costly dependency when forcing sort order

pagespec_match_list() makes the current page depend on the pagespec
being matched, so if you use [[!trailoptions sort="..."]] to force
a sort order, the trail ends up depending on internal(*) and is
rebuilt whenever anything changes. Add a new sort_pages() and use that
instead.
plugins/proxy.py: be compatible with Python 3
Merge branch 'ready/trail-sort'
Merge branch 'ready/comments'
Merge remote-tracking branch 'spalax/paternal/upload-svg'
Merge branch 'ready/more-proxy-utf8-fail'
Merge branch 'ready/perf'
Merge branch 'ready/postform-no'
more changelog and bug-closing