Bug fix allocation account

[csbarnet]

This PR addresses the issue reported here: #791
It also adds tests for this view.

A couple of things worth noting:
Although the main change was the change from 'request.is_ajax' to 'request.get("x-requested-with") == "XMLHttpRequest"', a couple of changes were made to enable testing.

I changed the way ALLOCATION_ACCOUNT_ENABLED was accessed from settings to enable testing. Originally, this value is assigned to a constant at the top of the file, so that it is only evaluated on import. However, this view is only accessible when this setting is assigned the non-default value. Using 'import_from_settings' in the view allows us to inject different values for the setting at test time using the 'override_settings' annotation.

I also changed the setup in the test for creation of the pi user to enable request.user.userprofile.is_pi to return True for a PI user.

[simonLeary42]

I have a nitpick: import_from_settings is typically used to declare a constant at the module level. Sometimes it's used to declare a constant in another scope, and other times it's used to set an object attribute. Very rarely is it used in any other way.

$ rg '= import_from_settings' | wc -l
175
$ rg '^[A-Z_]+ = import_from_settings' | wc -l
136
$ rg '^\s+self\.[a-zA-Z_]+ = import_from_settings' | wc -l
26
$ rg '^\s+[a-zA-Z_]+ = import_from_settings' | wc -l
10
$ rg --no-filename '= import_from_settings' | grep -vP '^\s*(self\.)?[a-zA-Z_]+ = import_from_settings' | wc -l
3
$ rg --no-filename '= import_from_settings' | grep -vP '^\s*(self\.)?[a-zA-Z_]+ = import_from_settings'
    context["SYSTEM_MONITOR_DISPLAY_XDMOD_LINK"] = import_from_settings("SYSTEM_MONITOR_DISPLAY_XDMOD_LINK", None)
    context["SYSTEM_MONITOR_DISPLAY_MORE_STATUS_INFO_LINK"] = import_from_settings(
# ALLOCATION_ATTRIBUTE_VIEW_LIST = import_from_settings(

note: I have an open PR that removes these two SYSTEM_MONITOR_* examples.