Code restructuring

[dteviot]

• [x] Move image handling code from BakaTsukiParser to BakaTsukiImageCollector.
• [x] Remove "BakaTsuki" prefix from Image class names.
• [x] Merge BakaTsukiEpubItemSupplier into EpubItemSupplier.
• [x] using "class" and "extends" to do inheritance.
• [x] Parsers should probably use Promise.resolve() in getChapterUrls()
• [ ] Replace chapters object with "FetchList" and chapter with "WebPage"
• [x] Replace XmlHttpRequest with fetch().
• [ ] Move the HttpClient fetch simulation from UtestWattpadParser into it's own file.
• [ ] Move code for splitting a web page's content into multiple EPUB chapters into its own class (as it's now used by 3 other parsers.)