Forms fixes: fieldset/legend, new .form-control-legend, accessibility advice

[patrickhlauke]

• Add new .form-control-legend class (to style <legend> like <label> for inline etc forms)
• More appropriate use of <fieldset> and <legend>
• Added accessibility advice for hidden/no label alternatives

Supercedes #17266

[patrickhlauke]

Note that even with this new pull request, the issue of having <legend> in the grid example is not completely solved.

However, the issue here is not so much the new .form-control-legend, but rather the fact that the right-hand side of the grid, for the radio button and checkbox cases, lacks the same top padding of the previous two rows (email, password). So, it's something that should be addressed there, rather than trying to force a specific vertical padding on the <legend> there, I'd say.

@mdo I would propose to merge this, and then address the vertical inconsistency in #19141 ?

[patrickhlauke]

@mdo thoughts on this?

[cvrebert]

LGTM, modulo adding a TODO regarding the legend alignment.

[patrickhlauke]

For completeness (as @cvrebert asked separately), this is how the <legend> on https://github.com/twbs/bootstrap/pull/19277/files#diff-582bb2b553026528f5f065d75a757c33R61 looks like after this.