Webscrapbook

* Fixed some issues of the previewer of the default theme of the web interface:
* Fixed possible code conflict after the previewer mode has been toggled repeatedly.
* Fixed layout error for rtl direction.

* Fixed token error during uploading through the default theme of the web interface when not hosted at root path.

* Improved the default theme of the web interface:
* Added `upload folder` command.
* Added support of uploading files and directories through drag-and-drop.
* Command selector is now disabled when a command is running.
* Moving a media in preview mode now shows a tooltip.
* Fixed zooming error for some SVG images in preview mode.

* Added MIME type support for WebP and WebM files.
* Fixed incorrect API messages.
* Improved the default theme of the web interface:
* Added a tools selector for operations like selecting, expanding, filtering, etc.
* Added new `Preview mode` in place of `List view`.
* Added new `Gallery mode (+media)`, and `Gally mode` now shows images without preloading media.
* Added support for more file types.
* Fixed path error in certain cases for commands.
* Improved the error messages for commands.

* Dropped support of config `app.base`.
* Fixed an issue that the mimetype of `.js` becomes `application/x-javascript` in some environment.
* Fixed several errors of unit tests.

* Fixed an issue that the sidebar scrolling point is reset when the tree refreshes.