Ebookmaker

- add logging for empty img[alt] attributes

- don't wipe source credit if there's an update credit from the db.
- require libgutenberg>=0.10.8

- tweak boilerplate to align with PG practice
- use `*** START OF THE` instead of `*** START OF THIS`. PG shifted from `THIS` to `THE` around 64000.

- changed caching strategy after analysis showed large cost for changes made in 0.12.12

- handle pre-html5 type attribute of `ul` properly
- continue processing job queue after a parse failure

- fix bug in removal of legacy elements
- add back enclose_text removed in 0.12.14; it wasn't redundant
- when an img is removed, replace it with alt text. fixes 123
- It seems we have to use the EPUB2 method to declare the cover image for EPUB3 files.