Add issues filters reset btn

What does this MR do?

Adds a button to the issues filter to reset all filters. I also lightly refactored a nearby method.

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

At the moment, the user can click the reset button even when no filters have been set. I didn't really like how it looked flashing the button on and off depending on whether a user had added a filter. I prefer having it there all the time, so the user is aware of it. But it wouldn't be difficult to hide if no filters have been set, if that's preferred.

Why was this MR needed?

So users could reset filters when they're filtering issues.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/14016

Screenshots (if relevant)

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