Add resize to trix editor

[mikiher]

Brief summary

Adds a resize handle to the rich text editor.

Which issue is fixed?

Fixes #3928

In-depth Description

• Added resize: vertical to text-editor style
• Set height and min-height properties (instead of max-height)

How have you tested this?

Resize handle now shows up where the rich text editor appears, and works as expected.

[advplyr]

The white resize handle doesn't look great but it works.

I remember a long time ago this used be an issue with Textarea but I think the browsers all use transparent handles now. I'm not sure what Trix is doing since there are no docs about it. It's not worth spending any additional time on right now.

Thanks!