Enable user pronoun editing from the admin ui

While users can edit their own pronouns, the admin UI enables an admin
to edit *any* user setting. As such, pronouns should be editable here,
too.

For the sake of simplicity, the input here is a simple text input field,
rather than a dropdown.

Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>

services/forms/admin.go

@@ -42,6 +42,7 @@ type AdminEditUserForm struct {
 	Website                 string `binding:"ValidUrl;MaxSize(255)"`
 	Location                string `binding:"MaxSize(50)"`
 	Language                string `binding:"MaxSize(5)"`
+	Pronouns                string `binding:"MaxSize(50)"`
 	MaxRepoCreation         int
 	Active                  bool
 	Admin                   bool