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.

modules/migration/comment.go

@@ -17,6 +17,7 @@ type Commentable interface {
 type Comment struct {
 	IssueIndex  int64 `yaml:"issue_index"`
 	Index       int64
+	CommentType string `yaml:"comment_type"` // see `commentStrings` in models/issues/comment.go
 	PosterID    int64  `yaml:"poster_id"`
 	PosterName  string `yaml:"poster_name"`
 	PosterEmail string `yaml:"poster_email"`
@@ -24,6 +25,7 @@ type Comment struct {
 	Updated     time.Time
 	Content     string
 	Reactions   []*Reaction
+	Meta        map[string]interface{} `yaml:"meta,omitempty"` // see models/issues/comment.go for fields in Comment struct
 }
 
 // GetExternalName ExternalUserMigrated interface