Allow admins / journal managers to update the ithenticate credentials

[ctgraham]

Describe the problem you would like to solve
Journal managers and administrators need to be able to change ithenticate credentials as needed. Currently, with the credentials only supported within config.inc.php, only system administrators with filesystem access can change these credentials.

Describe the solution you'd like
Add a settings form to the plugin, accepting the context-based (or site-based) username and password for ithenticate.

Who is asking for this feature?
Journal Managers, Journal Administrators, Technical Support

Additional information
n/a

[ctgraham]

Per #31 (comment)

Remaining task is:

I might suggest removing the ithenticate setting entirely, and instead checking to see whether there is a defined username in order to determine whether the plugin should operate...?