Allow using SelectedSubmodule in CustomCommands

[rlkandela]

PR Description

This PR allows the use of {{ SelectedSubmodule.Path }}, {{ SelectedSubmodule.Name }} and {{ SelectedSubmodule.Url }} in custom commands.

Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc

[stefanhaller]

Nice, thanks for the contribution. The code looks good (with one minor nitpick in the test). However, the commit history is a mess, please clean it up; see https://github.com/jesseduffield/lazygit/blob/master/CONTRIBUTING.md#commit-history.

[stefanhaller]

This change is unrelated to the rest of your changes, it should be a commit of its own (probably first). It is already possible to do this today, you just can't do much with it.

[stefanhaller]

That's the only line we have in that view, no need to navigate to it. We usually do

			Lines(
				Contains("submodule").IsSelected(),
			)

in cases like this.