Commit proxy images same (delayed) way as normally uploaded ones

[paskal]

Resolves #694.

Behavior before this PR

• Set EDIT_TIME=20s and IMAGE_PROXY_CACHE_EXTERNAL=true in docker-compose.yml
• Start dev instance using docker-compose, post comment with text ![](https://homepages.cae.wisc.edu/~ece533/images/cat.png)
• Restart docker-compose
• Wait for 2 minutes
• Reload the page: image from the comment will be re-loaded as it was stuck in Staging without commit and then was deleted after EDIT_TIME*5, 1m40s in this case. And with current logic, we re-load it in case it's not in storage.

Behavior after this PR

• Set EDIT_TIME=20s and IMAGE_PROXY_CACHE_EXTERNAL=true in docker-compose.yml
• Start dev instance using docker-compose, post comment with text ![](https://homepages.cae.wisc.edu/~ece533/images/cat.png)
• Restart docker-compose
• Wait for 2 minutes
• Reload the page: the image is still in place and loaded from storage.
  Message in logs:

  Commit image cached_images/da39a3ee5e6b4b0d3255bfef95601890afd80709-da39a3ee5e6b4b0d3255bfef95601890afd80709
  

[github-actions]

Pull Request Test Coverage Report for Build bc62a84b4-PR-701

• 9 of 15 (60.0%) changed or added relevant lines in 2 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.06%) to 83.542%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
backend/app/store/image/image.go	8	14	57.14%

Files with Coverage Reduction	New Missed Lines	%
backend/app/notify/notify.go	2	92.73%

Totals
Change from base Build 5579bd1a8:	-0.06%
Covered Lines:	5198
Relevant Lines:	6222

---

💛 - Coveralls