Consider implementing subscriptions feature as a form/modal

Nobody has claimed this yet.

Currently the subscriptions feature is all in the slash command `/github subscriptions add`, loaded with some unclear options through slash command flags, in particular the flag named `features`.

For example:

```
/github subscriptions add mattermost/mattermost-plugin-github --features pulls,issues,label:"Help Wanted"
```


Description of the feature from the command's help text:

- `/github subscriptions add owner[/repo] [flags]` - Subscribe the current channel to receive notifications about opened pull requests and issues for an organization or repository
- `flags` currently supported:
- `--features` - a comma-delimited list of one or more of the following:
- `issues` - includes new and closed issues
- `pulls` - includes new and closed pull requests
- `pulls_merged` - includes merged pull requests only
- `pulls_created` - includes new pull requests only
- `pushes` - includes pushes
- `creates` - includes branch and tag creations
- `deletes` - includes branch and tag deletions
- `issue_comments` - includes new issue comments
- `issue_creations` - includes new issues only
- `pull_reviews` - includes pull request reviews
- `label:` - limit pull request and issue events to only this label. Must include `pulls` or `issues` in feature list when using a label.
- Defaults to `pulls,issues,creates,deletes`
* `--exclude-org-member` - events triggered by organization members will not be delivered (the GitHub organization config should be set, otherwise this flag has not effect)
* `--render-style` - notifications will be delivered in the specified style (for example, the body of a pull request will not be displayed). Supported values are `collapsed`, `skip-body` or `default` (same as omitting the flag).

Start at the `/github subscriptions add` slash-command entry point and review how the `features`, `exclude-org-member`, and `render-style` options are currently handled. Define the form/modal scope and verify that it can configure the subscription choices and existing notification behavior described in the command help.

Written by the indexing model from the issue text.