Implement word-based content filtering

[rohnsha0]

closes #274

[mister-roboto]

@rohnsha0 thanks for creating this Pull Request and helping to improve Plone!

TL;DR: Finish pushing changes, pass all other checks, then paste a comment:

@jenkins-plone-org please run jobs

To ensure that these changes do not break other parts of Plone, the Plone test suite matrix needs to pass, but it takes 30-60 min. Other CI checks are usually much faster and the Plone Jenkins resources are limited, so when done pushing changes and all other checks pass either start all Jenkins PR jobs yourself, or simply add the comment above in this PR to start all the jobs automatically.

Happy hacking!

[rohnsha0]

Just noticed that plone.api was removed from dependency by @ale-rt... can't we depend on it....?
I have used its adapt_roles method... for elevated actions... or should I perform it using SecurityManager?

[rohnsha0]

@jenkins-plone-org please run jobs

[rohnsha0]

continued in #281