Updates screen: Plugin and Theme tables improvements

[afercia]

While investigating on #34774, noticed the Plugins and Themes tables in the Updates screen make use of th tags and scope attributes to establish relationships between the table cells. This is perfectly valid and must be done with tabular data. By the way I wouldn't say these are tabular data.

These tables are mostly used for presentation and it would be better to clean them up a bit to avoid screen readers reading out inconsistent relationships.
For example (see the screenshot below) marking the "Select All" cell as a th with a scope="col" attribute basically says the whole column contains data of type "Select All" :)
Screen reader read out this when moving horizontally through cells and users will hear something like:
"Select All Twenty Fifteen you have version..." when they're on the second cell.
The first cell (the one with the checkbox) is a td but has a scope="col" attribute which is invalid on a td.

TL;DR since this is a very simple table with just two columns and a (small) number of rows depending on how many updates are available, I'd suggest to remove any scope attribute and use just tds.
Any thoughts more than welcome.

[rianrietveld]

My 2 pennies:
In fact this is a table used for layout, semantically it seems more like a list to me.
Maybe remove the scope from the th and td first, and then think of a way to set up the layout, not in a table but in another, more semantic way.

[afercia]

First pass, the proposed patch removes scope attributes and changes table headers in simple table cells to remove any logical relationship between cells. Also, tries to simplify a bit the CSS.

[afercia]

Discussed a bit in ​the accessibility team chat a while ago and agreed to remove some semantics. While it may seem counterintuitive, in limited cases it's better to remove improper semantics (this is not a tabular data table) and noise for screen reader users and simplify all the things.
Worth noting also this screen will probably change soon as part of the Shiny Updates v2 effort currently under development on GitHub, see ​https://github.com/obenland/shiny-updates/issues/5

[afercia]

The patch also makes the h2 headings a bit bigger (default h2 font-size) to better separate the sections and removes the theme screenshot inline style. Screenshots before and after:

[afercia]

In 36477:

Accessibility: simplify the Plugins and Themes tables on the Updates screen.

Although it may seem counterintuitive at first, in very limited cases it's
better to remove improper semantics (this is not a tabular data table) in
order to reduce noise for screen reader users and simplify all the things.
Also improves headings to better separate sections.

Fixes #34780.