[Feature] Per-feed and per-category minimum polling delays

**Is your feature request related to a problem? Please describe.**
Currently, all feeds refresh at the same time — that is, when a refresh is triggered, all feeds are quickly queried sequentially. This can lead to rate limiting issues. I have seen these rate limits with YouTube and Spotify so far.

**Describe the solution you’d like**
I propose the ability to add a minimum polling rates for feeds. There's a couple types:
- Per-feed: Before the feed is polled, a delay occurs.
- This allows the user to specify a delay for a specific feed that may be within a category that doesn't need a delay between all feeds.
- Per-category: Set a delay for the group so that there is a delay that occurs between every feed in the group.
- For example: say one has a collection of YouTube feeds in one category. Setting this minimum delay would require a delay to occur between each YouTube feed in the category.

**Describe alternatives you’ve considered**
None.

**Additional context**
None.