Allow existing events to be linked to a channel

[fmartingr]

Right now only creating a new event allows for an user to link an event to a channel to get reminders, we need to let users link their existing events as well.

Acceptance criteria

• Work towards a reasonable UI/UX
• Remember the user linking the event that some of the event details will be shared out in the channel, where non-invited attendees could be.
• Allow users to search their events in order to link one to a channel
• Linked event channel should receive reminders 5minutes before the event takes place
• Linked event channel should receive event changes (if Allow event change notifications for Google Calendar mattermost-plugin-google-calendar#51 is done)

[hanzei]

Hey @fmartingr,

CRE would love to have this feature as it would also allow us to link your existing standup meetings to our team chat.

I wonder if a reasonable UI/UX could be a simple slash command that links to the current channel. Do you have ideas on how a user could pick the event to link? By URL?

[fmartingr]

That was the main problem for the UX, that you have to search for the event, and recurring meetings work different (you may use an ID for an instance of a recurring meeting, not for all as a whole). One idea would be to use a command to link an event by the link on the WebUI, but the URL in the webUI does not contain the event ID on it's own as well...

I mean, it's doable, but is not that straightforward. Would be happy to have this for our own meetings.