Prevent adding suspension with date in the past

[davwheat]

Fixes issue identified in QA

Changes proposed in this pull request:

Don't allow a date in the past to be used as a new suspension end date. Previously, if this was done, user would see a suspension notification without actually being suspended.

Reviewers should focus on:

Does this change any existing behaviour?

Not tested locally. Will do so after some more QA checking.

Confirmed

• Backend changes: tests are green (run composer test).

[askvortsov1]

Could we include automated tests for this? That would also add some coverage to general suspend functionality, and set up infra for future testing expansion.

[askvortsov1]

Willing to approve this without tests, but why "today" and not "now"?

[askvortsov1]

Hello, thank you for your pull request! In order to speed up future development, keep all work in one place, and take advantage of CI tools that can help us avoid breaking changes, we have moved all Flarum code to a single monorepo at flarum/framework. You can read more about this process in our dev diary. Unfortunately, pull requests can't be carried over to the monorepo, so we have to close all open pull requests. If this pull request is still relevant, please feel free to reopen it over on the monorepo. We apologize for the inconvenience, and hope you will consider contributing to Flarum again in the future.