Avoid showing very long pieces of text in notifications

Avoid showing very long pieces of text in notifications

I am unsure about this one. I am relying on `RichText#truncate_html`, to avoid having to put together my own truncation facility. My problem is that I find that method to be too coarse-grained as it works by removing entire HTML elements:

- If the text starts with a small paragraph, then a long one, it may only show the first.
- If it starts with a long paragraph, it may not show anything, or it will show the whole paragraph.

I think this is confusing. Note that can't just use Rails's own `String#truncate` as this is a `RichText`, not a `String`, and then we'd have to worry about truncating the HTML.

Having said that: `#truncate_html` is right there, and it is what we use for diary entries. So I guess let's reuse it, leaving it for a future contribution to rework it into something better, if there's inclination.