## Is your feature request related to a problem?

You can currently install plugins from Github and NPM sources, but not GitLab

## Describe the solution you'd like

Copy a GitLab URL to install a plugin

## Describe alternatives you've considered

Using the plugin source editor.

## Additional context

Came from a conversation in the Users' slack

#### *Thank you* for your feature request – we love each and every one!

posthog/plugins/test/mock.py
def __init__(self, text, status_code):
            self.text = text
            self.status_code = status_code
        def ok(self):
            return self.status_code < 300
    class MockBase64Response:
        def __init__(self, base64_data, status_code):
            self.content = base64.b64decode(base64_data)
            self.status_code = status_code
        def ok(self):
            return self.status_code < 300
    if args[0] == "https://api.github.com/repos/PostHog/posthog/commits":
        return MockJSONResponse([{"html_url": "https://www.github.com/PostHog/posthog/commit/MOCKLATESTCOMMIT"}], 200)
    if args[0] == "https://api.github.com/repos/PostHog/helloworldplugin/commits":
        return MockJSONResponse(
            [
                {
                    "html_url": "https://www.github.com/PostHog/helloworldplugin/commit/{}".format(
                        HELLO_WORLD_PLUGIN_GITHUB_ZIP[0]
                    )
                }
            ],
            200,
        )
    if args[0] == "https://registry.npmjs.org/posthog-helloworld-plugin/latest":
        return MockJSONResponse({"pkg": "posthog-helloworld-plugin", "version": "MOCK"}, 200)
    if args[0] == "https://github.com/PostHog/helloworldplugin/archive/{}.zip".format(HELLO_WORLD_PLUGIN_GITHUB_ZIP[0]):
        return MockBase64Response(HELLO_WORLD_PLUGIN_GITHUB_ZIP[1], 200)