Closes #1535

Closes https://github.com/gitlabhq/gitlabhq/issues/6745

Instead of using a fork, we subclass the filter and only apply the
`task-list` class to list items that actually are task lists.

Closes #1645

See https://github.com/github/task_list/pull/60

if something goes wrong, fall back to the plaintext lexer

Now they line up with their title.

Don't display the password editing form if the user's token is expired;
redirect to the form that allows users to request a new password reset
token.

Tell new users when their password reset token expires and provide a
link to get a new one.

Fixes #5825