Change the way we stitch MIME parts together for display

Right now, if a message contains multiple displayable HTML parts, we first concatenate all of them and only then process the resulting HTML (remove JavaScript etc). See https://github.com/k9mail/k-9/blob/e680c84085d82a0c1d0b06f10bf66ff7f42df907/k9mail/src/main/java/com/fsck/k9/mailstore/MessageViewInfoExtractor.java#L186-L257

This approach has lead to several clients being vulnerable to what is described as the Direct Exfiltration attack at https://efail.de/. Due to the way we handle encrypted content we're not vulnerable to EFAIL. But it's still very undesirable that the contents of a preceding MIME part can influence the way the following HTML part is parsed. So we should fix that by processing the different MIME parts individually and then stitching together the processed outputs.

Start with k9mail/src/main/java/com/fsck/k9/mailstore/MessageViewInfoExtractor.java, especially lines 186-257, to understand the current MIME-part processing flow. Change the handling so each displayable HTML part is processed individually before the processed outputs are stitched together; done means preceding MIME content cannot influence parsing of a following part.