Don't allow to create subprojects with same alias

[saadmk11]

Added a clean_alias method in the ProjectRelationshipBaseForm which will check if the alias already exists. This will prevent Projects from Creating New subprojects with same alias.
closes #5402

[stsewd]

Looks good, but we need to add some tests for this before merging.

[saadmk11]

@stsewd Okay. I'll add some tests and update the PR.

[saadmk11]

@stsewd I have Added some Tests and Updated the PR.

[stsewd]

Thanks for the tests, but we already have some of them here https://github.com/rtfd/readthedocs.org/blob/78c34c904b347110b2cd545b4b5a80ed526590f7/readthedocs/rtd_tests/tests/test_subprojects.py#L13-L13 can you move the new tests there?

[saadmk11]

@stsewd Thanks for pointing it out. I couldn’t find the existing test case as the name is different. I'll move the tests here and update the PR.

[saadmk11]

@stsewd Moved the tests to test_subprojects.py

[stsewd]

I think some tests are already there, but the test for the alias is correct 👍

[saadmk11]

@stsewd Should I remove the tests that are already there or this can be merged?

[stsewd]

You can try to identify the repeated tests, but it's fine for me.

[saadmk11]

@stsewd okay cool.

[saadmk11]

@stsewd Removed Repeated tests. :)

[stsewd]

Tested this locally, it's working