#17562 Fixes issues list filter only visible when user is logged in

## What does this MR do?

1. Moves the issue/MR list search form out of the `if current_user` conditional, allowing guests to view the field.

2.  Adds some margin to the search form, this is because in mobile view for guest users, the field was not supported by the RSS button and the New Issue button and would drop to meet the list.

## Are there points in the code the reviewer needs to double check?

Are there any views where the `margin-bottom: 12px` will affect the styling?

## Why was this MR needed?

Guest user UX. Nothing feels better than increasing intangible value!

## What are the relevant issue numbers?

Fixes #17562.

## Screenshots (if relevant)

![Screen_Shot_2016-05-16_at_23.48.06](/uploads/cf0f0c48287517ac3e83b204b4ee09b2/Screen_Shot_2016-05-16_at_23.48.06.png)

/cc @jschatz1 

**UPDATE:**

![Screen_Shot_2016-05-20_at_16.13.22](/uploads/eb854838a514d0d54d0f13bde1636b8a/Screen_Shot_2016-05-20_at_16.13.22.png)

See merge request !4171