Hey there.
I think the last time this was asked for was 12 years ago 
I don’t think it’s in the cards to expose this in the UI:
- As noted in this announcement, some big changes are coming to SonarQube, specifically around issue type and severity. I’m not sure severity will even remain something that is configurable outside of the rule definition.
- We really discourage the bulk activating of rules for “informational” purposes – while it can be well-intentioned (let’s make sure developers can see everything), it often serves as noise that actually discourages developers from paying attention to the issues that are raised (and that are actually important).
However, if you really want to, you can use the Web API.