Webscrapbook

* Bumped client requirement to >= 0.75.6.
* Bumped requirement of `werkzeug` to >= 1.0.0.
* Removed `cryptography` from installation requirement. It is now an optional requirement (for ad hoc SSL key generating).
* Fixed a bug for zip editing through server actions in Python < 3.7.
* Response 404 rather than 400 for `list` server action when the directory is not found.
* Added unit tests.

* Tokens are now created under `.wsb/server/tokens` instead of `.wsb/server/token`.

* Switched backend server framework to `Flask` from `Bottle`.
* Added support for `app.allow_x_*` configs to prevent issues when serving behind a reverse proxy.
* Dropped support for `server.ssl_pw` config.

* Added support of zip editing through server actions.

* Added support for `book.*.no_tree` config.

* Added `lock` and `unlock` actions.
* Merged `upload` action into `save`.