Investigate shipping Interventions for other Large Applications

[denschub]

The Slack User Agent override went well. We do have a couple of other large sites and large applications with known User Agent-based blocks on our list, so let's look into adding interventions to these.

Unfortunately, these cases are a bit more complicated than our usual interventions. In some cases, other teams are working with the site's developers to get things addressed, and we have to make sure to coordinate with them first to see if our actions would help that progress or not. Also, for things the size of Slack, we need to run internal QA on the interventions and be very careful not to make things worse for our users.

Based on these limitations, this warrants it's own task/card. Unfortunately, it's hard to estimate the total time for this. However, each individual interventions should not take more than two weeks of work time to handle.