Part of the [SAST Configuration UI][epic] feature, this implements form
submission behaviour, via a regular POST request. A future iteration
will replace this with a [GraphQL mutation][mutation].

Addresses https://gitlab.com/gitlab-org/gitlab/-/issues/223879.

[epic]: https://gitlab.com/groups/gitlab-org/-/epics/3659
[mutation]: https://gitlab.com/gitlab-org/gitlab/-/issues/227575