Issue 135 : Added ability to limit number of builds to keep (just like Jenkins)

[suratdas]

Addresses Visual-Regression-Tracker/Visual-Regression-Tracker#135
This pull request does following

1. Does not create separate images for baseline. It creates the reference to the actual image for baseline.
2. When deleting test run or test variation, it is first checked if it is in use by the other. Physical file is deleted only if it is not used by both variation and test run.
3. Since separate baseline files are not created, it saves disk space.

[pashidlos]

@suratdas thanks for contribution!

unfortunately this is not aligned with the idea of self contained entities
having one object implicitly related to multiple objects makes complicated it's management and could potentially lead to broken relations
I would sacrifice this additional memory consumption to simplicity in logic

[suratdas]

The logic has been modified to limit how many builds we want to keep just like Jenkins does without having us manually delete old builds. This is similar to issue Visual-Regression-Tracker/Visual-Regression-Tracker#146

[pashidlos]

could you add unit and/or e2e test for this feature?

[pashidlos]

LGTM!

[pashidlos]

@suratdas thanks for contribution!