Consider implementing subscriptions feature as a form/modal

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:<labelname> - 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).

mattermost / mattermost-plugin-github

Consider implementing subscriptions feature as a form/modal #780