[FrameworkBundle] make assets-install --relative equivalent to --symlink --relative

| Q             | A
| ------------- | ---
| Bug fix?      | kinda
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

The `--relative` option of `assets:install` only makes sense when asking for symlinks. Before this change, you were forced to pass the two options: `--symlink --relative`. This PR lets you just use `--relative` and the `--symlink` option will be set for you.

This PR also fallbacks to absolute symlinks when relative ones do not work.