Do not run new migrator if the main profile has been migrated

This is ephy *profile* migrator. It runs on a per-profile basis. i.e.
each web app runs migrators separately. So this migration step could run
once for a profile dir, then again far in the future when an old web app
is opened. But passwords are global state, not stored in the profile dir,
and we want to run this migration only once. This is tricky to fix, but
it's easier if we relax the constraint to "never run this migrator if it
has been run already for the default profile dir." That's because we don't
really care if a couple web app passwords get converted from insecure to
secure, which is not a big problem and indicates the user probably never
uses Epiphany except for web apps anyway. We just don't want all the user's
passwords to get converted mysteriously because he happens to open a web
app. So check the migration version for the default profile dir and abort
if this migrator has already run there. This way we avoid adding a new flag
file to clutter the profile dir just to check if this migrator has run.

https://bugzilla.gnome.org/show_bug.cgi?id=752738