Webscrapbook

* Fixed bad ID for item added by `wsb check --resolve-unindexed-files`.

* Dropped support of JavaScript when viewing a page archive, due to a security concern.

* Dropped support for `recursive` parameter of `list` server action.
* Added support of top-level None value for *.js tree files.

* Added unit tests.

* Added `convert wsb2sb` command.

* Bumped client requirement to >= 0.79.
* Adjusted locking mechanism:
* A lock is now created under `.wsb/locks` instead of `.wsb/server/locks`.
* A lock is now created as a file instead of a folder.
* A lock is now created with an ID, and can be extended using the same ID. Releasing a lock now requires its ID.
* The server now responses 503 rather than 500 for a timeout of `lock` server action.
* Added `cache`, `check`, and `convert` commands.