Pass the remember_me option into the u2f form and support it while authenticating

## What does this MR do?

Adds remember me support in the u2f authentication, and makes sure the flag gets passed from the login form to the u2f form.

Based on the changes for the same thing done for regular 2fa: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4369

## Why was this MR needed?

The remember me option wasn't working for u2f devices (yubikey)

## What are the relevant issue numbers?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/18103

See merge request !5918

Matches the changes done for non-u2f two-factor auth

17465 Fixed dropdown cursor key navigation

## What does this MR do?
This MR fixes the use of cursor/arrow/enter key events with search dropdowns, allowing a user to navigate up and down the list with the arrow keys and then select their item with the enter key.
It also applies some *minor* scroll user experience fixes, such as resetting the selected dropdown item every time it opens/closes (also stops multiple dropdowns conflicting) and forcing the dropdown scroll to scroll right to the top or bottom depending on whether they have selected the first or last item, respectively.

## Are there points in the code the reviewer needs to double check?
I would like someone with GitLab experience to check over whether this would harm any unique implementations of the GitLabDropdown or SearchAutocomplete.

## Why was this MR needed?
The current version has incorrectly behaving search dropdowns in the navbar, they either do not navigate using the keyboard or do not use the enter keystroke to select a highlighted item.

## What are the relevant issue numbers?
Fixes #17465.
Closes #20752.
Closes #21014.
**Contributes** to #20754.

## Screenshots (if relevant)
![17465.mp4](/uploads/1145abec226036abbaaa4aa46020f52b/17465.mp4)

See merge request !4781

Updated spec HAML