Previously, the WebAPITokens.objects.get_or_create_client_token would create

client tokens with no expiration date by default. In most cases, we want to

create client tokens with the client_token_expiration value set in the site

configuration (e.g. 1 year from now). Instead of carrying around the logic for

this to multiple areas, this change makes this value the default expiration

for WebAPITokens.objects.get_or_create_client_token.

Tested creating client/using client tokens with RBTool's login.
Ran unit tests.