Summary

When using a slash command that crashes a plugin, the user receives an unfriendly message.

Observed:

Command with a trigger of '/jira' not found. Click here to send as a message.

Expected:

Plugin for the jira command is not working. Please contact your system administrator.

Ticket Link

https://mattermost.atlassian.net/browse/MM-24753

Is your feature request related to a problem? Please describe.
Currently if you want to check a standup say a month old, it is difficult to check, you need to go through the entire chat history.

Describe the solution you'd like
We were looking if the standups can be sent as an email and also a way if i give a date I can get that day's standup.. Also a way to configure the emails on w

From another user I did /remind @amonnich "test" in 10s.

When receiving the reminder it showed the name of the other user correctly:

But after marking it as complete it shows my own name instead:

![image](https://user-images.githubusercontent.com/179599/77185330-87f65b80-6ad1-

From 5.24 MM server has a slash command autocomplete functionality feature. An API can be found here
To implement the feature add AutocompleteData struct to the Command when registering a slash command.

Similar PRs:
For Todo plugin mattermost/mattermost-plugin-todo#69

When someone is assigned to an issue/PR, we can use that event to create a post in Mattermost. Same for applying labels.

The task here is to add a new option two new options to /github subscribe, in order to create a subscription for these two event types.

Note that an issue/PR can have multiple assignees.