The recent work for retrofitting text-based file attachments for diff

support triggered some bad behavior in the commenting support.

TextCommentRowSelector and comment flag placement needed to work on a

table rather than a tbody. This caused mismatches between the element's

list of rows and the rowIndexes. It also caused a reference to an

invalid variable for comment placement.
One possible solution is to update TextCommentRowSelector to work with

tbodies, and a previous attempt went this route. However, future work

for diffviewer-style diffs for text attachments will require multiple

tbodies per table, so instead, this change splits the tbodies back out

into their own tables again.
The tables, for both plain text and rendered text, are split off into

reusable templates, reducing the amount of code duplication. The

revision information has been moved into these tables, under the tabs.

This will help with future review UIs that want to render plain text,

but also want a different UI entirely under the Rendered tab (such as

SVGs).
The revision label and selector rows are now hard-coded in the template,

as well, rather than being dynamically added later.
There's also some visual fixes for the page to prevent unwanted

gradients and to fix borders at the bottom of the view.

Tested reviewing individual revisions and diffs of plain text and Markdown

attachments, both source and rendered.
Verified in Chrome and Firefox that the visual styles were correct.