fix notification filters behaving weirdly

[connyduck]

This does four things

• set enablePlaceholders = false on PagingConfigs to avoid Paging Data that contains null placeholders, we don't want them (everywhere, not just in notifications)
• make sure NotificationsPagingAdapter does not crash when it encounters a null placeholder
• makes sure the notifications refresh correctly when the filters change
• the filters are now also respected when loading a gap

closes #4433

[connyduck]

meh now it sometimes makes more network requests than necessary

[mcclure]

I've been using this for a couple days now and I haven't noticed the issues from #4433 recurring. Will continue testing.