Report very slow on big courses

[timhunt]

REPORT_EDITDATES_ENABLE_FILTER_THRESHOLD logic was removed here 9ed935d#diff-828e0013b8f3bc1bb22b4f57172b019dL78. I needs to be put back.

[mspall]

@timhunt

I have created a draft pull request #41 with a potential solution to this issue. I don't have behat tests yet and might need some advice on those.

[mspall]

To give people using this plugin an update, we have had the code in pull request #41 on our production server since early November 2021. We have not had any issues and the complaints about the page being slow have gone to 0.