[UI] Use "(optional)" in labels to indicate optional textfields (#1571)

* Clarify optional fields in UI

Signed-off-by: Sunik Kupfer <kupfer@bitfire.at>

* Add icons to add webdav mount screen

Signed-off-by: Sunik Kupfer <kupfer@bitfire.at>

* Add title to group mount point address and name

Signed-off-by: Sunik Kupfer <kupfer@bitfire.at>

* Use assistant composable

Signed-off-by: Sunik Kupfer <kupfer@bitfire.at>

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Signed-off-by: Sunik Kupfer <kupfer@bitfire.at>
Co-authored-by: Ricki Hirner <hirner@bitfire.at>