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).

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

Just a sanity check should do.

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

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together