Prevent sprintf errors by escaping % in form title

[krokodok]

Fixes #1445.

Straightforward escaping of % in $post->post_title with %% prevents the error.

Checklist

• Project documentation has been updated to reflect the changes in this pull request, if applicable.
• I have tested the changes in the local development environment (see contributing.md).
• I have added phpunit tests.

Release Changelog

• Fix: Describe a bug fix included in this release.
• New: Describe a new feature in this release.

Release Checklist

• This pull request is to the master branch.
• Release version follows semantic versioning. Does it include breaking changes?
• Update changelog in readme.txt.
• Bump version in stream.php.
• Bump Stable tag in readme.txt.
• Bump version in classes/class-plugin.php.
• Draft a release on GitHub.

Change [ ] to [x] to mark the items as done.

[kasparsd]

Please see this comment on the other pull request #1485 (review) -- please let me know if I've misunderstood anything.

[krokodok]

@kasparsd Tried to complete the % escaping, analogue to #1485.

[krokodok]

Superseeded by #1508.