Change logic to enable/disable loan extend button

_Originally posted by @kprzerwa in https://github.com/inveniosoftware/react-invenio-app-ils/pull/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.

Start at the patron profile page's current-loans list and trace the existing conditions controlling the “Request extension” button. Then inspect the REST serializers to determine where the condition results can be exposed; it is done when the backend returns the needed flags and the button uses them consistently.

Written by the indexing model from the issue text.