campaigns: large changesets can result in 413 errors

Created by: LawnGnome

We've addressed this to some extent with #9202 (closed), but this is still an issue: creating a large campaign (for example, with lots of noise in vendored packages or lockfiles) can result in 413s.

The work going on in #12333 (closed) will (significantly) improve this: each changeset spec will be uploaded individually, which means that we essentially move from the size of the campaign being the issue to the size of the changeset, but this doesn't completely remove the potential for 413s.

In the longer term, we should provide a GraphQL mutation that can accept partial changeset specs, and make src-cli smart enough to know when to chunk specs before transmission.