Add missing tag icons

[pascalwittler]

The tags "Wagen", "Wagengattung" and "Fahrzeugnummer" do not have an icon which results in

• less space left for the text input value when creating/editing the tags (compare [1] and [2])
• a less beautiful and inconsistent appearance in the list (see [3])

[1] Large enough text input thanks to the icon on the left

[2] Too small text input due to a word as icon fallback

[3] Inconsistent appearance of tags with and without icons