Disallow editing when no fields are editable

fixes #376 on github

Default the 'Edit' button to be disabled with explanatory title text. on js initialization, search the DOM for edit fields, which are only present when fields are in fact editable, and if they exist, re-enable the 'Edit' button.
Merge pull request #469 from steventlamb/bug/376_disallow_editing

Disallow editing when no fields are editable