MM-23044: Bootstrap channel export plugin and add a dummy exporter

[agarciamontoro]

Summary

This PR bootstraps the channel export plugin and adds the initial structure to start development.

For now, the plugin does one thing: it exposes one command, /export, through which the user can "export" the channel they execute the command in. Once the export is ready, a bot sends the file to the user. For the moment, the exported file is just an empty JSON file with name <epoch-timestamp>_<channel-ID>.json.

• The first commit, afbe545, has no important modifications. It only adapts the README from the starter template and make the necessary changes to plugin.json, as well as adding mattermost-plugin-api as a dependency. It also deletes the webapp directory, which won't be of use in the near future. The third commit, 8c99d86, just adds the CI configuration.
• The important commit here is 43a998b: it ensures that the bot exists when the plugin is activated and it adds the command /export, which asynchronously triggers the export process (this the dummy part) and get the bot to send it to the user once it's finished.

EDIT after reviews:

1. First review
   • I addressed all minor reviews in f9221e4, 324f6d8, 6bcf988, 4ca5b9f.
   • The important structural changes are isolated in 6c13637. See comment MM-23044: Bootstrap channel export plugin and add a dummy exporter #1 (comment) for more information.

Ticket Link

https://mattermost.atlassian.net/browse/MM-23044

[lieut-data]

Nice work, @agarciamontoro! Really appreciate the high-level description of the changes, as well as the discrete commits.

A few comments inline.

[agarciamontoro]

So I changed the structure here a bit. What I want is:

1. that the user receives an ephemeral post as soon as they execute the command, letting the export and upload of the file run concurrently, as this process may take some time
2. decouple the communication with the user and the export itself. I will work further on this in my next PR.

For now, the structure is as follows:

• executeCommandExport is the owner of the communication with the user. It sets everything up (the channel to export and the channel to send the DM to), sending an ephemeral post in whether there's an error or the export process has begun successfully. The export process (namely, export the channel and upload the file) is triggered from a goroutine that still lives inside executeCommandExport, and that is in charge of informing the user of any error occurred during the export process, or to create the post with the successfully exported and uploaded file.
• The exportChannel and uploadExportedChannelTo are the functions in charge of doing the real work. The former will absolutely be modified in the next PR, and will probably be something more complex than a simple function, in order to allow different exporters. The latter will probably remain similar, but I still need to investigate if it makes sense to design this so the exporting and the uploading is concurrent, so may change too.

[lieut-data]

Thanks, @agarciamontoro!

[alifarooq0]

Nice work @agarciamontoro 🎉!

[agarciamontoro]

@prapti, I assigned you here to keep you in the loop, but I guess that a QA review at this early development phase may be redundant, as things are likely to change soon (in my next PR, for example). So feel free to mark it as done and we can outline a test plan to do when this is a bit stable.

[lieut-data]

Agreed, @agarciamontoro -- let's loop @prapti in when we're ready to cut a release.