LP#1939162: add option to clear added content cache to Angular staff catalog

LP#1939162: add option to clear added content cache to Angular staff catalog

This patch restores the ability to clear the added content cache
to the staff catalog by adding a "Clear Added Content Cache" item
to the Other Actions menu on the Angular staff catalog record page.

As with the former embedded staff catalog, clearing the added content
cache does not result in the staff catalog attempting to automaticaly
reload the cover image; the user will still need to reload the page
to see if the change has taken effect. Improving that behavior would
be for a separate bug.

This patch also includes a fix to tag an unrelated string for
localization.

To test
-------
[1] Ensure that at an added content handler is active.
[2] Install the patch.
[3] Navigate to a bib record in the Angular staff catalog.
[4] Observe that there is a Clear Added Content Cache item
    in the Other Actions menu.
[5] Click it and note that a toast stating that the cache was
    cleared is displayed.
[6] (extra credit) Test while making changes to the record (say
    to the ISBN) that would result in a different cover image
    being provided by the added content source.
[7] (extra credit) Update opensrf.xml to remove the added content
    handler, restart services and Apache, then try the cache-clearing
    action. This time, toast should be displayed stating that the
    action failed.

Release-note: Adds a "Clear Added Content Cache" item to the Other Actions menu in the staff catalog record page.

Signed-off-by: Galen Charlton <gmc@equinoxOLI.org>
Signed-off-by: Jason Boyer <JBoyer@equinoxOLI.org>
Signed-off-by: Galen Charlton <gmc@equinoxOLI.org>