Form demo / themeTester: font sizes unequal

[bill]

​form.html demo and ​themeTester.html have unequal sizes for the fonts of the input widgets. Same for many of the tests.

This is due to [11765], which added

body .medium {
        font-size: larger; /* help test widget sizing */
}

to dijitTests.css.

It's good for testing but messes up the demos. Should either:

• remove that rule
• don't include dijitTests.css into the demos (but then couldn't switch themes automatically)
• etc.

[bill]

(In [13447]) Make themeTester.html have enabled and disabled versions of all the form widgets. Refs #6152, #6574, #6087

Also removed class="medium" affecting sizes on themeTester. Refs #6326.

And did some general cleanup/refactoring of tabs.

[bill]

(In [13721]) Remove definition of "medium" from dijitTests.css since it messes up demos. Fixes #6326.