change config option to enable bbcode Check via a flag

[gersona]

Proposed changes

BbCodeCheck disabled by default and enabled via bbcode-text flag

Checklist

• Lint and unit tests pass locally with my changes.
• I have added tests that prove my fix is effective or that my feature works.
• I have added documentation to describe my feature.
• I have squashed my commits into logic units.
• I have described the changes in the commit messages.

Other information

• fixes Consecutive duplicated words false positive with bbcode markup #8172

[nijel]

docs/user/checks.rst needs to be updated with the changed behavior (you can generate the header using ./manage.py list_checks), versionchanged block should be added there as well.