Support importing comment types (#22510)

This commit adds support for specifying comment types when importing
with `gitea restore-repo`. It makes it possible to import issue changes,
such as "title changed" or "assigned user changed".

An earlier version of this pull request was made by Matti Ranta, in
https://future.projects.blender.org/blender-migration/gitea-bf/pulls/3

There are two changes with regard to Matti's original code:

1. The comment type was an `int64` in Matti's code, and is now using a
string. This makes it possible to use `comment_type: title`, which is
more reliable and future-proof than an index into an internal list in
the Gitea Go code.

2. Matti's code also had support for including labels, but in a way that
would require knowing the database ID of the labels before the import
even starts, which is impossible. This can be solved by using label
names instead of IDs; for simplicity I I left that out of this PR.

models/issues/comment.go

@@ -175,6 +175,15 @@ func (t CommentType) String() string {
 	return commentStrings[t]
 }
 
+func AsCommentType(typeName string) CommentType {
+	for index, name := range commentStrings {
+		if typeName == name {
+			return CommentType(index)
+		}
+	}
+	return CommentTypeUnknown
+}
+
 // RoleDescriptor defines comment tag type
 type RoleDescriptor int