Make the use of parameter variables consistent

[Jaifroid]

In line with https://github.com/kiwix/kiwix-js/pull/467/files#r247341333, global parameters used in the app (and often stored in cookies) could be used more consistently, and we could centralize the functions that set UI elements that depend on them. This can be done, for example, with a global params object, that could be defined at the top of app.js, or perhaps in init.js (as is the case in Kiwix JS Windows).

[Jaifroid]

We already have a global params object, it's just that we don't use it consistently throughout, and we use a number of other variables to set the UI.