Prevent tag modal from disappearing

* In Chrome ~32 the modal would report as display block, and using the
inspector everything looked correct, except that the content was
invisible. Toggling display:block on and off brought the content up.
* Debugging showed that the <legend> element caused the strange
behaviour, the fix is to remove when using a modal.
* I can’t find any Bootstrap or Chrome bugs that relate to this.
Merge pull request #183 from alphagov/fix-tag-modal

Prevent bulk tag modal from disappearing when selecting many paths