changeset_templater: backport parts map of [templates] section from formatter

formatter: add support for parts map of [templates] section Unlike a mapfile whose template is looked up by spec -> mapfile -> topic, [templates] section is global. We use :sub-section syntax to define parts per template.

changeset_templater: backport separator template from formatter As commented, this should be used with docheader and docfooter, not with header nor footer. That's one reason why no props are passed to templater when rendering a separator. (See map-cmdline.changelog to understand what the "header" is.)

formatter: add support for separator template This seems useful for writing JSON template.

formatter: add support for docheader and docfooter templates templatepartsmap() is a minimal copy of changeset_templater.__init__(). I tried to factor out a common function, but it was unnecessarily complicated.

formatter: extract helper function to render template

changeset_templater: do not enable verbosity postfix for [templates] section Since this postfix hack exists only for backward compatibility, we don't need it for new [templates] section. This isn't a BC as templates defined in [templates] section weren't loaded until recently.

changeset_templater: simplify handling of verbosity postfix

merge with stable

tests: don't touch the network when using virtualenv