[exhentai] rename metadata fields to match API results (#1325)

- gallery_id    -> gid
- gallery_token -> token
- title_jp      -> title_jpn
- visible       -> expunged
- gallery_size  -> filesize
- count         -> filecount

Also changes the function of the 'metadata' option.
It is now boolean and causes extra data fields from the API to be added
instead of completely replacing the data from HTML when activated.

docs/configuration.rst

@@ -927,14 +927,15 @@ Description
 extractor.exhentai.metadata
 ---------------------------
 Type
-    ``string``
+    ``bool``
 Default
-    ``"html"``
+    ``false``
 Description
-    Select the gallery metadata source.
+    Load extended gallery metadata from the
+    `API <https://ehwiki.org/wiki/API#Gallery_Metadata>`_.
 
-    * ``"api"``: Get data from the `API <https://ehwiki.org/wiki/API>`_.
-    * ``"html"``: Extract data from HTML.
+    Adds ``archiver_key``, ``posted``, and ``torrents``.
+    Makes ``date`` and ``filesize`` more precise.
 
 
 extractor.exhentai.original