Make the search block cacheable

We've been using this patch, but it's had the interesting side-effect of showing other users' unrelated search queries in cached search blocks. Need to dig into this a bit; maybe a custom cache context would help.

Is that via the url params? if so we can add `url.query_args:%key` cache context where %key is the query param name from the url

@larowlan Thanks, I'll give it a go.

We're using the MR with cache context in production right now. Works really well and haven't seen any issues with cached search queries.

Adding a bit more to the description. I want to take a look at config changes before we look at RTBCing -- might be difficult to merge given how much else is coming through.