This commit fixes the documentation of settings as we have
replaced "Your account" section into two new sections -
"Profile" and "Account & privacy".
This commit also fixes a comment in the test for settings
documentation in test_middleware.py.
Because our mention logic now has a way to distinguish a user with one of these
weird names from a wildcard mention, we no longer need this explicit warning.
(It'd still be a bit confusing, but no more so than various other weird names you could
invest).
This changes the border-radius to 6px for the tabbed display, which is not
in line with the current Zulip style for border-radius (4px). However 6px
really looks a lot better for this (possibly because it's a bigger box than
most of our other boxes?)