make SeriesBookmarks show unread chapters only
properly remove series bookmark

Make it similar to removing chapter bookmark: actually removing the
series item out of the array instead of setting its state to "removed"
like before.

Also show "removing" status on button.
give series descriptions unique keys

to suppress react unique key warning
refactor series_bookmarks

Split different parts of SeriesItem into separate functions for clarity
show "no new chapter" msg in series-bookmarks