Added UI elements to switch between mobile and desktop version.

I don't actually use the mobile app, but one thing that really annoys me in mobile apps it the inability to access the desktop version. So I added UI elements to switch between desktop and mobile versions while respecting navigation. Though a few pages are missing direct navigation (namely DMs, mentions, hashtags).

Since it is now possible to switch manually from one to another, I have added automatic redirect of mobile and desktop clients in the index.html file.

Note that I do not believe the button inside the Desktop app to be responsive compatible as it is accessed through the drop-down menu.

I have not actually tested this in a mobile browser, if some kind soul wants to do that and give their +1 here before merging it would be the best thing to do.

Also feel free to tell me I'm an idiot and that no one wants this, really I'm just throwing it out there to see what people think.