invent: introduce template for the Extragear projects

Currently only master branch gets the builds because extragear projects
are special and doesn't have specific naming pattern for the stable
branch. Or even specific policy, there are projects with

• numbers 1.1
• project name and version (Krita/1.x)
• master branch itself behaving as stable
• no stable branch, just tags

And this is so confusing for templatifying the Gitlab CI jobs, It seems
to me that project might have to define stable jobs themself in their
yml file. It is best way to deal with problem