Improve embed credit line

Gets the text closer to the spec:
* links have #mediaviewer (lots of code duplication, see #373)
* image has alt text (was title in the spec but that made less sense)
* less convoluted logic in getCreditHtml()

Change-Id: I43db84adb7fe29850706f92ee978016939b59aaa
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/369
Merge "Improve embed credit line"