add accessibility switch (and accessibility mode)

[bansp]

This issue references the hub issue #317 and is the first logical fork in the process of taking acceptability requirements seriously: can we introduce an accessibility mode, with a parallel display, or should we modify the current display? I can see pros and cons of both, and at a hunch, I would prefer to keep the current layout and work on adding acceptability features on a side, as a mode. I simply like our current aesthetics, but it's obvious that we should also address users with impairments, the more so that we are marketing SIS as a service for, among others, the end users of CLARIN (and ideally also other) repositories.

Being able to switch to the accessibility mode at a click of a button would make it possible for us to tinker with the display on a side, without modify the current, not fully accessible, state. The most important question then is: is an introduction of an accessibility switch treated as an acceptable step towards enhancing acceptability? Is there a standardised way of installing such a switch, so that user agents can recognise it and switch modes automatically for users who need that mode to be on from the start?