3.1.1

Feature changes:

• Add book stats graph and refresh. By @webofpies in 154 and 162.
• 138: Separate Word Count and % Known into separate columns and support sorting.
• Allow term listing search in translations.
• 155: Add "words per page" field during book creation. By @fanyingfx.

Bug fixes:

• 112: show different options if backup failed (retry, skip, adjust settings)
• Sort statuses properly in the term listing.
• 95: editing pages updates book word count
• Shorten migration file names to prevent Windows file path length exceptions

Misc back-end:

• Add term Export CSV test.
• Calc book stats on at least 5 pages.
• Clean up some form styles.
• Speed up book stats calculation.