Issue title: laggy ajax response causing unintended behavior
 Issue body: when I'm using the keyboard to scroll through one of the lists, I'm noticing some odd behavior that seems like it's caused by lagging ajax responses. It's most noticeable in the feed/category list, but it happens in the item list as well. 

The problem happens, for example, if I'm in a category at the top of the list and I want to key down to one toward the bottom of the list. As I key through the categories/feeds in between, it sends an ajax call to retrieve the data for each of them. Then, when I get to the final intended category, it often renders the list for one of the feeds that I had keyed past. 

It seems like you could probably alleviate this (and cut out a lot of unintended calls to the api at the same time) by putting a delay on the request. Instead of making a request as soon as a feed is active, you could wait 200ms or so to make sure the user had actually  intended to make that request.

 Comments: 
 Comment 0: Another solution would be to `.abort()` ongoing requests before placing the next one.

 Comment 1: Yeah it would be good to do something about this.

Leaning towards @Zegnat's suggestion of just canceling the request.

 Comment 2: Driving me crazy ... a fix would be very much appreciated :)

 Comment 3: Feedbin will now `abort` the request for a feed if another request is made.

{'source': 'GithubIssues', 'repo_name_with_owner': 'feedbin/support', 'issue_id': 14568382, 'repo_stars': 86.0}