Adjust CSS / HTML of Workflow Transitions page

A few minor adjustments to improve page layout

• new CSS class (.form-container .footer), similar to .submit-button but
  without centered text
• use <h2> tag for section titles instead of a table row
• move "who can change" selects within the corresponding sections
• display form submit button centered
• delete button within a section of its own

Note: the 2 buttons are not displayed within a single section because it
would generate invalid HTML (closing the main form within an open div or
require nested forms).