[MM-54770] Add ability to export profile pictures and fix importing them (#25042)

* add ability to export pp and fix import

* remove unused nopSeeker

* remove debug log

* fix shadow vars

* generate a warning instead of an error when unable to export profile picture

* fix merge conflicts

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Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/app/imaging/decode.go

@@ -113,7 +113,7 @@ func (d *Decoder) DecodeConfig(rd io.Reader) (image.Config, string, error) {
 // GetDimensions returns the dimensions for the given encoded image data.
 func GetDimensions(imageData io.Reader) (int, int, error) {
 	cfg, _, err := image.DecodeConfig(imageData)
-	if seeker, ok := imageData.(io.ReadSeeker); ok {
+	if seeker, ok := imageData.(io.Seeker); ok {
 		defer seeker.Seek(0, 0)
 	}
 	return cfg.Width, cfg.Height, err