Since ff2f31b LibWeb has segfaulted when clicking on links, as the
browsing context (a GCPtr) in the lambda was captured by reference
and was out of scope by the time the callback fired.
This commit moves the regular handling of links to the anchor elements'
activation behavior, and implements a few auxiliary algorithms as
defined by the HTML specification.
Note that certain things such as javascript links, fragments and opening
a new tab are still handled directly in EventHandler, but they have been
moved to handle_mouseup so that it behaves closer to how it would if it
was entirely up-to-spec.
