Add smv_build_targets configuration option. This will allow users to
specify an arbitrary number of build targets to build for each version
of the documentation, and specify whether it should be available to
documentation consumers to download. This supports all sphinx builders,
and output formats where there is a single output file. For build
targets where there are multiple output files (such as html), all archive
formats supported by shutil.make_archive are supported. Downloadable
build targets are placed in an `artefacts/` directory within each
versions build directory.

Add smv_clean_intermediate_files configuration option. This will allow
users to determine whether the intermediate files are removed or not.
This is a useful configuration option where the generated build
artefacts are not for public download consumption.

Reorder build arguments passed to `sphinx` such that the source and
build directory are the first two arguments. This is to ensure that
there is compatability between the `-M` and `-b` sphinx build flags,
which have slightly different APIs.

Add apathto method to discover artefact paths.

Add dictionary comprehension to pair build target names and artefact
paths to each version object. This will ensure that the build artefacts
for the current build is accessible via the current_version key in
html_context.

Add modifier to replace slashes in branch names with hyphens for file
naming purposes. This will ensure that the file name is not split and a
truncated version used for the artefact name.

Add shutil.py as a third party library to ensure backwards compatability
can be achieved with python 3.6. shutil.copytree only accepts the
dir_exists_ok argument from python 3.8.

Add unit tests and update metadata schema with appropriate mocked
objects for app configuration.

Add documentation detailing appropriate configuration of the
smv_build_targets configuration setting including exemplars to
illustrate the features and also limitations of this feature.

Add documentation detailing appropriate configuration of the
smv_clean_intermediate_files configuration setting.

Add exemplar usage of the new artefacts object in the html context
within template files.

Closes sphinx-contrib#70.

Add file matching that accounts for project names that include spaces
and is case insensitive. This is to match how latex and other builders
generate their artefacts.

Use only the project name from the build configuration rather than the
configuration on each branch to account for when the project name has
been changed to display differently.

Refactored pattern matching to only match patterns when there's more
than one result. When there's one result, this is assumed to be the
build artefact, which should provide some flexibility for artefact
recognition without sacrificing quality. This also opens the door for
future filtering based on user specified patterns.

Trying to access a download artefact from a page other than the root
would fail to resolve the correct artefact path. This change adds
support to resolve the correct path from any page the user is located
on.

Add unit tests to confirm this feature works as expected, and also to
ensure artefacts are found correctly when the project name contains
spaces and variable casing.

…l context

* Add feature to ensure that build targets with downloadable=False don't
  populate the artifacts html context. This fixes a bug in which setting
  downloadable=False would provide a download link with the provided
  HTML jinja templates. Setting downloadable=False will now no longer
  result in a download link being made available.