The `rollup-plugin-license` has been using too many deprecated dependencies, so it is necessary to remove it.
As an alternative, this changes uses Rollup `output.banner` to insert copyright information. Since `terser` runs after `output`, it is not possible to insert the Front Matter defining permlink for `sw.js` through the same way (Jekyll Front Matter is YAML rather than JS, which would cause errors with terser).
Therefore, _Jekyll Collection_ is now used to add permlink to `sw.js`, with the collection named `app`, and the directory placed in `_app`. This directory is not tracked by git, but it will be included when building the gem.
When using the theme starter, which uses a git submodule,
assets loaded from that submodule are "untracked" files in the pages branch working tree.
The build can fail at this line due to Git, saying "the following untracked working tree files would be overwritten by checkout".
Adding `-f` forces the checkout, overwriting all files.
The other deploy steps then take over and rewrite the files as needed.
