MC: Make Description required, decrease Environment text area height

Discussion started here: https://mojangab.slack.com/archives/G0NM0QG14/p1544228263004600

Since some of the latest Jira updates the Environment field is higher than it used to be. This often makes users (and even sometimes us helpers and moderators) accidentially write the description in the Environment field.

We noticed that MCPE has the Description as required field. It would be great if you could do that for MC as well.

Additionally it is possible to reduce the size of the Environment field without manipulating the CSS, see https://community.atlassian.com/t5/Jira-discussions/Environment-field-in-the-new-version-is-confusing/td-p/586907 (found by Neko). This would as well help preventing this problem.