MM-11424 Extend bulk import to support themes across teams(#9305) (#9419)

* MM-11424 Extend bulk import to support themes across teams(#9305)

Also added:
Advanced Settings:
+'feature_enabled_markdown_preview'
+'formatting'
Sidebar Settings:
+'show_unread_section'

* MM-11424 (PR review) user teams' theme validator test updated

* MM-11424 (PR review) added test with valid JSON of invalid theme(by structure)

JSON string contains numeric and JSON object fields, not just a correct "string":"string" map

model/preference.go

@@ -18,6 +18,7 @@ const (
 	PREFERENCE_CATEGORY_ADVANCED_SETTINGS   = "advanced_settings"
 	PREFERENCE_CATEGORY_FLAGGED_POST        = "flagged_post"
 	PREFERENCE_CATEGORY_FAVORITE_CHANNEL    = "favorite_channel"
+	PREFERENCE_CATEGORY_SIDEBAR_SETTINGS    = "sidebar_settings"
 
 	PREFERENCE_CATEGORY_DISPLAY_SETTINGS = "display_settings"
 	PREFERENCE_NAME_COLLAPSE_SETTING     = "collapse_previews"