[Feature] Change .onclick handlers to addEventlistener

**Is your feature request related to a problem? Please describe.**

Using .onclick is generally considered undesirable. I seem to have overlooked them in #2234.

**Describe the solution you'd like**
Replace by addEventListener.

**Describe alternatives you've considered**
Leaving it as is, but see https://github.com/FreshRSS/FreshRSS/pull/3901 for why that's probably not the best idea. People will unsuspectingly copy it.

**Additional context**
There are others like .onerror.