Mattermost-server: Helper function to build path to plugin asset

[mattermod]

Plugins may expose custom assets via the assets directory, but must then combine the SiteURL and their plugin ID to build the resulting path. Let’s simplify this in the plugin helpers with a GetPluginAssetURL(asset string) that builds a URL to the requisite resource.

Note that this is distinct from the GetBundlePath present in the API, that returns a filesystem path to the asset in question.

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JIRA: https://mattermost.atlassian.net/browse/MM-16819

[Lumexralph]

@mattermod @hanzei I would like to work on this.

[hanzei]

Thanks @Lumexralph :+1:

[Lumexralph]

@hanzei I need to ask some questions, who should I reach out to?

[hanzei]

Me :wink: How can I help?

[Lumexralph]

@hanzei Can I reach out to you in the Mattermost workspace? It would be convenient for me, hope it's okay by you?