Casetracker actions autocomplete doesn't assign to uid

Issue Summary

If the casetracker_actions assign to action is selected and there are a lot of assignable users, the user select list is replaced by an autocomplete. Where the select list returns values in the form:

After submitting, the case will be assigned to anonymous because a non-numeric uid (username instead of uid) was entered. This patch solves the problem during submit by checking for a numeric value and searching for a valid username => uid replacement. I'd prefer it if the autocomplete was fixed, but this works fine too. (see patch)