Refactor search controller for consistency and clarity

[dhruvkb]

This is a revision of PR #699 formed by reverting commit 7e009fd and then rebasing against main.

Original description

This PR refactors the search controller, splitting it up into smaller, easier to understand chunks with proper documentation and type hinting.

This PR

• removes frivolous arguments to search, using the MediaSearchRequestSerializer as the single source of truth for search params
• applies the excluded provider check only when there are non-zero excluded providers
• updates the get_sources function to use DSL instead of raw query to ES
• updates usage of Search to be idiomatic with Elasticsearch DSL conventions
• moves globally accessible ES connection to settings.ES
• reduces code duplication

Testing Instructions

This is a refactor, passing CI should be sufficient proof that no functionality is impacted.

Checklist

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• I added or updated tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no visible errors.

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[obulat]

Approving as we have already approved this PR before :) Really hope it works now that the API is throttled for unauthorized requests.

[sarayourfriend]

As I was last time, I am still uncomfortable with the size and scope of this refactor. Even though I do agree it seems to make things much more uniform and consistent. I'm not sure that for me it makes anything clearer; perhaps for someone already familiar with the code it does though!

I left one suggestion that might help clarify at least a small part of the code. I think a lot of the code is pretty cryptic to begin with and I hope we can refactor/add tests/add comments to help with that in the future.

If there is absolutely any way to break up this refactor and to deploy it in steps, I think that would be a benefit, but I can appreciate that that is not always possible.

[sarayourfriend]

This is interesting, I didn't know we did this. I wonder if circumventing ES's built-in load balancing could be an issue for us.

[dhruvkb]

I don't know enough to comment if it can be a problem, but I can see how redirecting all requests from a particularly active consumer to a single shard can be a bottleneck. I haven't modified it in this PR though, it's been that way from before.

[dhruvkb]

I'm going to merge this so that we can unblock #776. It's very huge and unwieldy as-is so it's better for any changes to this to be addressed in faster, smaller, easier-to-review PRs.