Application Link creation shouldn't have limitation of using Bamboo Base URL

Issue Summary

When configuring application link between bamboo and any atlassian cloud server product, Bamboo Base URL needs to be used even if bamboo is behind a firewall and not direct accessible from outside.

Steps to Reproduce

1. Configure application link between Bamboo and (for e.g.) Jira Cloud
2. Navigate as Bamboo Admin to Cog >> Overview >> Manage Apps >> Application Links.
3. Create Cloud application link for a product by entering the URL that you use to access the remote application.
4. When a creating a reciprocal link for Bamboo, you need to use the Base URL of Bamboo that you use to access the remote application. In case Bamboo is behind a firewall, you cannot use a different URL than the Base URL, as while proxy will rewrite http redirects, it probably won't rewrite base url in payload.

Expected Results

When creating application links, Bamboo should be allowed to use other than Bamboo Base URL for example Microsoft application proxy.

Actual Results

Currently we can use only Bamboo Base URL while configuring application links.

Workaround

None.