Fix some accessibility issues

[hannaeko]

Hello, I am trying to address some of the accessibility issues raised in #998, #1001, #1002 and #1003.

Changes:

• replace div by main for .container element
• change about page title to be "About | nitter.net"
• add role, label and title for liberapay icon / link
• use explicit label to link label to input (this also changed the way checkbox are styled, from a span directly following the checkbox to styling the checkbox directly)
• use separate fieldsets to group inputs in preference form

Cheers,
Gaël

[untitaker]

this patch is now deployed on nitter.woodland.cafe -- lighthouse a11y score seems to marginally improve on tweets, so I consider that a win