Anki add-on: Fix Invalid Field HTML

If you find that note content appears outside of field boxes when editing a note, this is caused by invalid HTML in your fields - likely as a result of pasting from external sources in Anki 2.0, or from manually editing the HTML in AnkiDroid. This add-on will attempt to rewrite your HTML, which in the common case of unbalanced div tags, should fix display problems. There may be invalid constructs that get worse when being rewritten however, so you should manually verify any cards you selected are displaying correctly after using the add-on. You can use it by selecting one or more notes in the browse screen, then using the Fix Invalid HTML menu item from the Notes menu. Please use caution when using this add-on : Use File>Export to export a backup of your entire collection prior to making any changes. After using the add-on, review any cards you selected to ensure they display correctly. If you notice any problems, you will need to restore from the backup you made.