Add ability to select a source other than GitHub when adding in a Source Location

...on a component's definition page.

To replicate:
* Go to https://clearlydefined.io/definitions/pypi/pypi/-/zuul/3.3.0
* Click on the pencil next to "Source Location"
* The pop up will only feature GitHub as a place to add the source url from

Better workflow:
* At the very least, allow GitLab to be used as a Source URL
* Consider adding a field to put a custom source URL into should the code be hosted somewhere other than GitHub or GitLab

Start on the component definition page and inspect the Source Location pencil dialog, which currently offers only GitHub. Define and implement support for GitLab and a custom source URL, then verify the dialog accepts and displays each supported source location.

Written by the indexing model from the issue text.