Add original plurals and labels to row preview plural strings

[pedro-mendonca]

For plural strings, currently the row preview only shows the translation text plurals, the original text only shows the singular.

On translate.wp.org template both singular and plurals are shown in the original text, with "Singular" and "Plural" labels:

[pedro-mendonca]

The editor should have the same layout for consistency:

<small>Singular:</small>
<span class="original">Original text...</span>

The Locales which have 3 to 5 plural forms have both Originals on top, being the text areas below.
These Locales benefit the most from this plural labels being in the same layout, and for the consistency to be perfect, the blockquote top margin of the current Translation text should also be set to 0.

Currently it has a bad readability: Plural form labels inline and textarea label with huge margin.

Suggestion: Move the labels above, as it's on the row-preview, and use the same 0 margin between the label and the text.