Add new providers to the harvest page

[bduranc]

Issue Type: Feature Request

The current harvest page doesn't accurately reflect the new harvest sources recently added (for GitLab, Gradle plugins, and Go packages).

For example, if you go to the page and select Git(Hub) as the harvest source... how do you differentiate harvesting a package from GitHub vs. GitLab? The text boxes don't seem to have any indication of this, nor do the selection buttons themselves.

The same can be said about harvesting from the Gradle plugins provider that was added recently.

For Go components, there isn't even an option to select this even though they are now supported.

The harvest page is a key part of the overall user experience and should be updated so users can conveniently harvest from these new locations.

Similar issue #917