Fix several issues in view feed of com_tags component

# PR Summary

This PR fixes issue #4623 and several coding errors which I found while reviewing code in view feed of com_tags component:

1. The getListQuery method of TagsModelTags component doesn't return "author" and "author_email", so there are notice errors in the following lines of code in view feed:
https://github.com/joomla/joomla-cms/blob/staging/components/com_tags/views/tags/view.feed.php#L54
https://github.com/joomla/joomla-cms/blob/staging/components/com_tags/views/tags/view.feed.php#L72

2. $item->displayDate in the line of code https://github.com/joomla/joomla-cms/blob/staging/components/com_tags/views/tags/view.feed.php#L55 need to be replaced with $item->created_time (there is no field displayDate in the $item object)

3. $item in the following lines of code need to be replaced by $feeditem

https://github.com/joomla/joomla-cms/blob/staging/components/com_tags/views/tags/view.feed.php#L68
https://github.com/joomla/joomla-cms/blob/staging/components/com_tags/views/tags/view.feed.php#L72
4. $row->author_email needs to be replaced by $item->author_email
https://github.com/joomla/joomla-cms/blob/staging/components/com_tags/views/tags/view.feed.php#L72

# Testing instructions:
For some reasons, these notice errors are not displayed when I tried to view the feed, so it will be difficult for end-users to test this pull request (these errors are logged in error logs of the web server). Maybe one of PLTs (or developers who understand Joomla code) can help reviewing this code and get it merged? 

For end-users, the only think you can test is:

View rss feed for tags component. You will see that for each feed item, the author and pubDate tag are not available for each feed item.
Apply this pull request. Check it again and you will see that author and pubDate are available now for each feed item.

As much as it hurts me to suggest this right now: The tags have a TagHelperRoute class, which creates the right URL, which needs to be called instead of the static URL handed over to JRoute. Please take a look at that. Otherwise you get the wrong Itemid for this.