Mattermost-server: Use CTRL/CMD + Alt + K to insert markdown link formatting

[mattermod]

When a user has text selected in the message composition areas (center pane or right-hand side) and hits CTRL + Alt + K* (Windows) or *CMD + Option + K (Mac) then insert the link markdown formatting around the selected text:

[selected_text]()

Note after using the shortcut, the cursor should be placed inside the round brackets so the user can immediately paste a link.

There was a similar PR done for bold and italics formatting that can be used as reference: https://github.com/mattermost/mattermost-webapp/pull/6230

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JIRA: https://mattermost.atlassian.net/browse/MM-30274

[esethna]

@jp0707 let us know if you're interested in working on this as it's similar to the PR you submitted for bold and italics :) https://github.com/mattermost/mattermost-webapp/pull/6230

[jp0707]

@esethna Yup! I will work on this.