support altering quality without specifying format

Status: **Opened for visibility, DO NOT MERGE**

Reviewers: **@jansepar**
Linked PRs: **(links to corresponding PRs, optional)**

## Changes
- Adds handling of the quality parmater in image resizer requests in the absence of a format parameter (previously it would be dropped)
- Don't merge before changes to backend are shipped

## How to test-drive this PR
- Take a look at the tests