accounts: allow setting exact expiry date

set with a text input field which uses the same date parsing library as
the search function. Parsed expiry date will appear once you've typed
something in, so you can make sure it's right.

models.go

@@ -261,11 +261,12 @@ type customEmailDTO struct {
 }
 
 type extendExpiryDTO struct {
-	Users   []string `json:"users"`               // List of user IDs to apply to.
-	Months  int      `json:"months" example:"1"`  // Number of months to add.
-	Days    int      `json:"days" example:"1"`    // Number of days to add.
-	Hours   int      `json:"hours" example:"2"`   // Number of hours to add.
-	Minutes int      `json:"minutes" example:"3"` // Number of minutes to add.
+	Users     []string `json:"users"`               // List of user IDs to apply to.
+	Months    int      `json:"months" example:"1"`  // Number of months to add.
+	Days      int      `json:"days" example:"1"`    // Number of days to add.
+	Hours     int      `json:"hours" example:"2"`   // Number of hours to add.
+	Minutes   int      `json:"minutes" example:"3"` // Number of minutes to add.
+	Timestamp int64    `json:"timestamp"`           // Optional, exact time to expire at. Overrides other fields.
 }
 
 type checkUpdateDTO struct {