Add /autolink import/export capability

[levb]

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#75 implemented most of the backend functionality to support this feature. Before it can be merged, we would like to add the capability to upload/save local files. It can be done with client-side commands, see https://github.com/mattermost/mattermost-plugin-jira/blob/6bb4e25bbef08a43b26716626382f518f3e28df1/webapp/src/hooks/hooks.js#L47 for an example.

See also #71 that introduced the config read/write capability.

[adarj]

Hello, I'm interested in contributing this feature. Just to confirm, the import and export commands should allow the user to upload/download config files from their local machine, correct?

[levb]

@adarj Welcome and thank you for taking this on!

RE: "allow the user to upload/download ... from local machine" - correct, but if that proves to be difficult to do directly, 0/5 an acceptable first implementation would be to export/import to mattermost files (uploaded as attachments for import, and for export, posted as a new message, with an attachment)

Also would be cool to have a "merge" capability in the import!

[levb]

@adarj I am going to re-label this as a "Up For Grabs" to have someone else add the requested "File Import/Export" UI feature. It seems like an essential addition we should have before merging the existing PR. They can be implemented with client-side /commands.

I will explicitly request that it is done as a continuation of your PR, since most of the backend code is already there and you should get the attribution for it.