Intercept issues search form submit to preserve filters.

## What does this MR do?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/896 by intercepting manual search form submission and redirecting it to use existing logic (now factored out into `executeSearch`).  

## Why was this MR needed?

Manual form submission (keying in 'enter') in issues search did not preserve applied filters. 

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/896

See merge request !6054