Bug 1484071 - Crop large data URLs before displaying them in the status bar r=Felipe

This commit provides a simple fix to reduce the time spent
rendering larger data URLs (such as large screenshots) in
the status panel when the Highlights tab is hovered over.
By limiting the number of characters displayed, this
decreases the time spent in the _showDelayed function.

Differential Revision: https://phabricator.services.mozilla.com/D21593