Serve plugin icon

Serves the icon for {RepoName}, whether or not that plugin is installed. Resolved in three tiers: 1. a local icon.png in the plugin's directory; 2. the iconURL out of the plugin's own installed pluginInfo.json; 3. the iconURL out of the listed pluginInfo.json for {RepoName}, for plugins that are listed but not installed (the Browse tab). Serving these same-origin is what lets the Plugins page show icons at all -- raw.githubusercontent.com is allow-listed in FPP's CSP connect-src, not img-src. The client passes only a repo name: tiers 2 and 3 both run their URL past PluginIconURLIsAllowed() before fetching, so the set of things this endpoint can be made to request is bounded by the plugin list and by that allow-list, not by the caller.