Webscrapbook

* Fixed a compatibility issue for saved tree data with old browsers not supporting ES2019.

* Fixed a packaging error for 0.44.0.

* Added support of `single_file` format for `items` converter.
* Added support of more common MIME types.
* Fixed translation of certain common MIME types to a fixed file extension.
* Fixed several get_meta_refresh errors in Python 3.6.
* Fixed several test errors.

* Added support of custom MIME types mapping using `~/.config/wsb/mime.types`.
* Added `wsb help mimetypes` for documentation about customizing MIME types mapping.
* Added support of some common MIME types.
* Reworked handling of some HTTP headers and HTML attributes to conform with the spec better.
* Fixed an issue that meta charset be ignored when parsing a meta refresh.
* `items` converter now preserves file metadata for the generated HTZ or MAFF files.
* Fixed incorrect icon path after a conversion of the `items` converter.
* `wsb2sb` converter no more changes the linefeed format for output files.
* Fixed potential bad handling of XHTML files for the `wsb2sb` converter.
* Fixed incorrect rewriting of content in special tags like `<template>`, `<xmp>`, etc., for `migrate` and `items` converters.

* Added support of migrating several older WebScrapBook data for the `migrate` converter. The behavior can be switched with `--convert-*` options.
* `migrate` converter no more changes the linefeed format for output files.
* Adjusted log message format of several utilities.
* Fixed missing support of hash in source URL for the generated static index file.
* Fixed a potential error when converting a legacy ScrapBook ID with the `migrate` converter.
* Fixed an issue of generating extra loader elements if one exists for the `migrate` converter.
* Fixed potential bad handling of XHTML files for the `migrate` converter.

* Bumped server requirement to >= 0.6.
* Now use `save` instead of `upload` action to upload a file.