Fix nuxeo-diff-page header when comparing doc with long title

When using Compare (either docs or versions) on a Document with a long title, the header text is cut off at the borders and overflows slightly onto the Search icon.

Steps to Reproduce:

1. Create a Document with a very long title.
2. Either create a version out of the above Document or create a new Document with an equally long title.
3. Run either Compare Versions from DOCUMENT_ACTIONS or Compare from RESULTS_SELECTION_ACTIONS accordingly.

Expected Result: nuxeo-diff-page header content is displayed without cutoffs (e.g. ellipses used to shorten long titles)

Actual Result: Document titles are not shortened, header content is cut off at the borders.