Change logic to enable/disable loan extend button

[kpsherva]

Originally posted by @kprzerwa in #65

In the patron profile page, in the list of current ongoing loans, the button "Request extension" is enabled/disabled depending on some conditions. The idea is to move all conditions, if possible, to backend. The results should return for example:

{

... is_too_early: true/false

}

This can be achieved via REST serializers.