Replace anchor elements with button elements

[bjarnef]

In Umbraco core many accessibility changes have been many.
One of them is replacing <a> (anchor) elements with <button> elements for buttons unless it is actually a link, but most are not.

It is also ideal to specifify type attribute since browsers may have different default behavior of <button> elements.
Most of the time <button type="button"> can be used.

Furthermore for icons using <i> element, it is also useful to specify aria-hidden="true".

[nul800sebastiaan]

Thanks @bjarnef, I've put it on the list of things to look into when we're ready to work on accessibility issues.

[AndyButland]

These have been updated where identified (i.e. as noted in #107) and all purely decorative icons using <i> have had the aria-hidden="true" attribute added. Due in the upcoming releases.