hooks: on Git, invoke hooks/post-receive-custom from hooks/post-receive

Make it possible for admins to install all kinds of hooks.

Based on a patch by Tim Ooms.

A more generic solution would be nice, but for now we aim for this minimal
solution.

hooks: clarify in UI and documentation how the hook control essentially is Mercurial only

The Git hook model doesn't directly allow multiple hooks of the same kind.

py3: officially support Python 3

All tests pass and no known regressions.

docs: update links to Mercurial's website and wiki

Also Mercurial itself is not a coding standard, so add possessive suffix.

docs: cleanup of casing, markup and spacing of headings

Mostly stuff found and fixed by Søren, extracted here to separate things.

Other uses of title casing might be debatable, but here it was just a few
documentation headings that clearly were inconsistent with the majority.

docs: rework stuff

The existing docs were far from how we wanted it to be. There was so much to do
and it is not feasible to do that cleanup it in clean patches.

Instead, I took a sweep through the docs and changed what I thought could
benefit from a change: structure, examples, advices, language, markup, content,
etc.

First step in two-part process to rename directories to kallithea.
This first step is to change all references in the files where they refer
to the old directory name.

fixes #753 inform about some problems with antivirus software in RhodeCode
on windows