Only channel admins should be able to create subscription

[mickmister]

At the moment, any user can create/edit/delete GitHub subscriptions. This functionality should be limited to users with permission to manage the given channel. The following cases should allow a user to edit subscriptions:

• The channel is a private channel and the user has PERMISSION_MANAGE_PRIVATE_CHANNEL_PROPERTIES for this channel
• The channel is a public channel and the user has PERMISSION_MANAGE_PUBLIC_CHANNEL_PROPERTIES for this channel
• The channel is a direct message (DM)
• The channel is a group message (GM)

_Issue created from a Mattermost message by @hanzei._

[GibranHL0]

Hi 👋

I'd like to be assigned to this issue and give it a try.

[hanzei]

Awesome, thanks @GibranHL0 :tada: Please let me know if you have any questions.