10395: Improve textarea elements

[Mwindo]

This PR:

• Disables textarea resize from horizontal scrolling
• Prevents textarea resizing to smaller than one line of text
• Consolidates two existing and inconsistently applied CSS classes into one consistently applied CSS class

[Mwindo]

Are we CERTAIN that all textareas should have this? Is this a sensible default to have?

I ask as we have added styles to ALL tables and ALL forms which should not exist. I would only add a style to all elements if 99% of the usages need that style, anything less and it should be passed in.

This is a good question. Given the ticket description and uses of textarea, I think this is a case in which we want the style 99+% of the time. That said, I think there is probably room to clean up our textarea classes (e.g., the existing ustc-message-modal-text-area is just resize: vertical, and there really shouldn't be a need for that now). Let me take care of that.

In the process, I'll consider using a separate CSS class, which I do generally prefer, rather than a default style. The argument for the default here is that it really is a default, and it doesn't touch the page layout like margin (etc.) does. The argument against is that in the future this could be wrong; also, without a separate class, a future developer might be tempted to throw something like margin into the default style.

UPDATE: I saw two similar existing CSS classes -- ustc-message-modal-text-area and resize-vertical-only -- that did part of what we wanted (resize: vertical), but they seem to have been inconsistently applied and maybe forgotten about. Therefore, instead of using a default style, I kept the "existing approach" but with only one CSS class -- textarea-resize-vertical -- applied consistently.

[Mwindo]

I've made a few small changes and am happy to address feedback on them, but for now I'm going to merge