Use CircleCI secure context when uploading bundle to Backblaze (#1755)

Resolves tiny-pilot/tinypilot-pro#1260

<s>We need to make the following changes in CircleCI:
1. Create a CircleCI secure context for TinyPilot Community.
1. Open the [TinyPilot CircleCI
context](https://app.circleci.com/settings/organization/github/tiny-pilot/contexts)
    1. Click "Create Context"
    1. Enter a context name:
        * Perhaps we should name it `community-production`?
    1. Click "Create Context"

1. Restrict the TinyPilot Community context to only be used on the
`master` branch and when SSH is disabled on the CI job.
    1. Click the TinyPilot Community context
    1. Click "Add Expression Restriction"
    1. Enter the following expression in the text box:
        * `pipeline.git.branch == "master" and not job.ssh.enabled`
    1. Click "Add Expression Restriction"

1. Add new production Backblaze credentials to the TinyPilot Community
context
* [Follow these
instructions.](https://github.com/tiny-pilot/tinypilot-pro/blob/a8c064b6d60929c23d16e7042a0b8a15600ff42b/docs/cycle-service-credentials.md#backblaze-application-key-for-tinypilot-community)</s>

After we've merged this PR, we can remove the `BACKBLAZE_*` environment
variables from the [TinyPilot Community CircleCI project
settings](https://app.circleci.com/settings/project/github/tiny-pilot/tinypilot/environment-variables).

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.circleci/config.yml

@@ -306,6 +306,8 @@ workflows:
           requires:
             - build_bundle
       - upload_bundle:
+          context:
+            - production-tinypilot-community
           requires:
             - verify_bundle
           # Uploading a new bundle affects Gatekeeper's view of the latest