Add available currencies in the plugin config

[markusahlstrand]

Currently it's possible to select from a set of currencies when locking an article, but typically a publisher only uses one currency. It would be great to select the currencies available in the plugin config and to not show the currency picker on an article if it's only one currency enabled.

[markusahlstrand]

Move the currency picker from the post editor and add it to the plugin settings.

[markusahlstrand]

Also show the current currency in the post settings

[markusahlstrand]

Also remove the currencies from the pass editor