www/pelican: Prefix according to Python conventions

• Rename (move) according to Python package naming conventions [1]
• Switch temporarily to GITHUB for source files until missing content files for the test suite is merged/fixed [2]
• Add python (virtual category) to CATEGORIES
• Add LICENSE_FILE
• Add OPTIONS for Markdown and Syntax Highlighting, they are OPTION'al features
• Add MARKDOWN and SYNTAX to OPTIONS_DEFAULT (for package convenience).
• Use OPTIONS helpers for TESTS option.
• Refactor TEST_DEPENDS so it only declares tests_require, moving the RUN_DEPENDS assignment to TESTS_BUILD_DEPENDS.
• Rename TEST option to TESTS to match other Python ports, and tweak description to canonicalize
• Deprecate OPTIONSFILE override (no longer necessary)
• Canonicalize regression-test target, so it gets the setuptools wrapped setup.py arguments for proper 'test' command support.
• Re-order makefile sections (DEPENDS|USES|OPTIONS)

While here:

• Add MOVED entry
• Update www/Makefile

[1] https://wiki.freebsd.org/Python/PortsPolicy
[2] https://github.com/getpelican/pelican/pull/1793

Reviewed by: jlaffaye (maintainer)
Approved by: jlaffaye (maintainer)
Differential Revision: https://reviews.freebsd.org/D3328