Generated test archive templates: migrate old uses, new option for markdown samples.

[isoos]

• version name containing legacy is no longer in use - removed
• random-based repository url is replaced with the template-based parameter
• for repository and homepage, and empty string will make the field absent
• added option to include markdown_samples.md as part of the readme (with the most common markdown format items, to be extended later)
• added markdown samples in templates_test, to track regression in markdown parsing
• added markdown samples to browser test to track it screenshots (attaching the desktop view)