Ebookmaker

- add whitespace to `<br>` elements in headers. tidy used to put whitespace around `<br>` elements. PPs relied on this behavior, so we're stuck.
- add pagebreak css for EPUB2 boilerplate
- changed log level for empty img[alt] from ERROR to WARNING
- fix replacement css for table[align] and img[align]
- fix incorrect css for bgcolor attributes
- fix incorrect css for clear='all'
- changed the approach to replacing `<center>` - there's additional css for tables in a center element.
- the HTML void element `wbr` is not recognized by lxml. for this reason, we had to
- switch to `html.parser` for files likely to be html5 (not xml)
- we're using `html.parser` for files that don't set `xmlns` attribute and don't declare a PUBLIC doctype `-//W3C//DTD`, which should be any non-xml HTML5 file. Only 151 files in our collection satisfy this criterion so far.
- explicitly remove stray end tags: `</wbr>` after writing to out bytes for the html5 file
- add svg cover from 132
- remove 'svg' property for stand-alone svg files; this property is meant for files that embed svg.

- 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