support cross-repository screenshot mask

### Describe the problem

`freedomofpress/securedrop` stores its screenshots in `freedomofpress/securedrop-docs`. We don't want to store them directly in the `securedrop` repository. Instead, we'd like to be able to set the screenshot mask of `securedrop`'s Weblate component to the corresponding mask within the `securedrop-docs` repository.

### Describe the solution you'd like

We've thought of three options:

1. **Support internal (`weblate://`) urls in the `Component.screenshot_filemask` field,** so that we can add `securedrop-docs` as a component (whether or not actually translated) and refer to it that way from the `securedrop` component.
2. **Support submodules,** so that `securedrop` can pull in `securedrop-docs` without containing its Git objects directly.
3. **Support Git LFS,** so that `securedrop` can pull in `securedrop-docs` without containing its file content directly.

### Describe alternatives you've considered

_No response_

### Screenshots

_No response_

### Additional context

Our use-case is described in freedomofpress/securedrop#6968.