Allow back-dating user creation via the `adminCreateUser` API operation.
`CreateUserOption` now has an optional field `created_at`, which can
contain a datetime-formatted string. If this field is present, the user's
`created_unix` database field will be updated to its value.

This is important for Blender's migration of users from Phabricator to
Gitea. There are many users, and the creation timestamp of their account
can give us some indication as to how long someone's been part of the
community.

The back-dating is done in a separate query that just updates the user's
`created_unix` field. This was the easiest and cleanest way I could find,
as in the initial `INSERT` query the field always is set to "now".

Run `make generate-swagger` and update the comment describing the field
so that they make sense from an API standpoint (and not just when reading
the Go code).

Thanks @yardenshoham for spotting this.

Co-authored-by: Yarden Shoham <hrsi88@gmail.com>

Since any timestamp is accepted by the system, avoid mentioning
"back-dating" user creation.

With this change to the "insert" query, it was possible to set the creation
timestamp:

```diff
@@ -740,11 +747,11 @@ func CreateUser(u *User, overwriteDefault ...*CreateUserOverwriteOptions) (err e

 	if err = DeleteUserRedirect(ctx, u.Name); err != nil {
 		return err
 	}

-	if err = db.Insert(ctx, u); err != nil {
+	if _, err = db.GetEngine(ctx).NoAutoTime().Insert(u); err != nil {
 		return err
 	}

 	// insert email address
 	if err := db.Insert(ctx, &EmailAddress{
```

Detect whether `NoAutoTime()` should be used based on `u.CreatedUnix`,
and add two unittests that test the new functionality.

…-user-creation

Suggestion by @wolfogre

Co-authored-by: Jason Song <i@wolfogre.com>

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