Use U.S. English spelling for 'optimization'.

[davidangel]

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[orlangur]

1. We must not change code of third-party libraries (like KnockoutJS)
2. Changing fieldset name is backward-incompatible as to me: http://devdocs.magento.com/guides/v2.2/contributor-guide/backward-compatible-development/index.html#magento-apis

[orlangur]

@davidangel, thanks for your contribution, it looks like all places which could be changed are already fixed in #11345.

[davidangel]

1. Reverted. Is there a reason knockout.js isn't included via npm?
2. I was not able to find any other instances of search_engine_optimisation in the codebase. I was able to find 7 other results for search_engine_optimization. Are we sure this change is not desired?

[orlangur]

Is there a reason knockout.js isn't included via npm?

Yes. We don't want NodeJS on prod. It could be included via some Composer plugins but for now it works like that.

Are we sure this change is not desired?

Yeah, third-party customizations are relying on names. Keeping them unbroken is lesser evil than a typo in caption not shown to end user.