Improve accessibility in site profiles

[marcus-herrmann]

While ProcessWire's core site profiles aren't as central to PW as other core themes to other Content Management Systems ProcessWire should at least inspire developers to make their ProcessWire websites more accessible.

Both "core themes" of WordPress and Drupal (8), Twenty Sixteen and Bartik lead themers by good example, accessibility-wise. ProcessWire should do this as well, in my opinion.

The changes in this merge request do not affect the visual output of the core site profiles, but:

• their behaviour when using a site profile via keyboard, by adding a "jump to content" bypass block
• their output towards screen readers, especially regarding the search form (I added labels since placeholders should never be used as form input labels and changed the way of hiding elements)
• their semantics (main, aside, role-attributes)