Mattermost-server: Support goto_location on mobile

[mattermod]

When a slash command replies with a goto_location parameter, the mobile app should launch the user’s browser and send them to that location. The following semantics apply:

• If the url is absolute, but starts with the currently configured server URL, treat it as a relative URL. Otherwise, open the browser to that URL.
• If the url is relative and points at a channel name (including a DM or GM), then open that channel.
• If the url is relative and not handled above, then open the browser to that URL.

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JIRA: https://mattermost.atlassian.net/browse/MM-27123

[larkox]

I already had a tackle on this, but forgot to update the Jira issue. Here is the PR: https://github.com/mattermost/mattermost-mobile/pull/4620