Bump version to v3.1.0

[gemmaleigh]

3.1.0

• Split the list of partials imported by GOV.UK elements into two further files - elements and frontend-toolkit. This supports npm-sass, where the frontend toolkit dependencies are imported separately (PR #489).
• Add a new class .body-text for use inside legends, for text to accompany radio buttons or checkboxes - either 'select one', or 'select all that apply'. Ensure legends or elements within legends have margins in webkit browsers (PR #484).
• Move the breadcrumb so it sits outside the main element (PR #478)
• Constrain error summary boxes to 2/3 of the page width (PR #477)
• Remove the right padding from the last column of items in a table, also remove color set for table headers and cells, to allow users to change colour settings (PR #482
• Align table captions to the left (PR #476).
• Add guidance for use of table captions (PR #488).
• Fix unnecessary float and width 100% for .form-section and .form-group (PR #487).
• Fix incorrect margin above the last panel in a group (PR #498).
• Add guidance for the mininum text size to be used with the highlight box on confimation boxes (PR #481).
• Update govuk_template_jinja to 0.22.0 (PR #493).