Fix incorrect MediaLibrary links when base path is not the domain root

[adsa95]

This PR fixes #4039, a bug that was introduced in PR #3536.

Currently the app root/base path is added to the storagePath variable in MediaLibrary, which is then used to generate the links with the getPathUrl() function. Since PR #3536 this is then also passed through the Url::to()function to adhere to the link policy, however the base path is added again in this method and therefore we get duplicated base paths in the final URL (only noticeable if your base path is not the domain root, and not using S3 etc). This PR solves the issue by not adding the base path to storagePath in the MediaLibrary.

storagePath is a protected variable and not used anywhere else within the MediaLibrary so this should not cause any other problems, it would however affect anyone extending the MediaLibrary class and using this variable.

Unfortunately I have no experience using tests, and not the time needed to learn this within a reasonable timeframe for the need of this PR. Ping @w20k

[LukeTowers]

@w20k could you put together a test case for this?

[w20k]

@adsa95, could you look at #4065, seems like there is one more issue with path duplication... Odd one! Thanks 😉