Admin: Use constant with Akismet when used

[kraftbj]

Incomplete:

• I'm not sure yet how to disable the form field and hide the key when using the constant (to match Akismet's behavior). We can't assume an administrator is supposed to be able to see the key.

Fixes #8226

Changes proposed in this Pull Request:

• Use the API key as provided by Akismet (allows for a filter and a constant).
• Needs UI to not render the form field if set via a constant (and maybe not show if using a filter?)

Is this a new feature or does it add/remove features to an existing part of Jetpack?

• No.

Testing instructions:

• Add key per instructions in UI doesn't handle Akismet keys set via constant properly #8226 on a site with a plan.
• Visit Admin, search for spam. See the Akismet settings field.
• Before patch, it says you need a key. After patch, says you are protected.

Proposed changelog entry for your changes:

• Admin Dashboard: Use Akismet API keys set with a constant or filter.

[jetpackbot]

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Scheduled Jetpack release: May 14, 2019.
Scheduled code freeze: May 7, 2019

Generated by 🚫 dangerJS against 95ccd95

[stale]

This PR has been marked as stale. This happened because:

• It has been inactive in the past 3 months.
• It hasn’t been labeled `[Pri] Blocker`, `[Pri] High`.

No further action is needed. But it's worth checking if this PR has clear testing instructions, is it up to date with master, and it is still valid. Feel free to close this issue if you think it's not valid anymore — if you do, please add a brief explanation.

[stale]

This PR has been marked as stale. This happened because:

• It has been inactive in the past 3 months.
• It hasn’t been labeled `[Pri] Blocker`, `[Pri] High`.

No further action is needed. But it's worth checking if this PR has clear testing instructions, is it up to date with master, and it is still valid. Feel free to close this issue if you think it's not valid anymore — if you do, please add a brief explanation.

[jeherve]

closing in favor of #16542.