Feature/migrate GitHub to existing framework

[Tguntenaar]

Resolves #265

Still a WIP

Testnet Preview

Acceptance Criteria:

• GitHub plugin aligns with the new framework (Introduce community plugin management framework #253). So an admin can enable, configure, or remove the plugin in the community settings page.
• If a community admin enables the GitHub plugin, the GitHub tab shows up on the community page navigation.
• Admin can configure the plugin within the community settings page, not from the old GitHub tab. Remove any instances from the GitHub tab to configure the page from the tab.
• If a community admin enables the GitHub plugin, they can click configure to customize the the tab name as well as the existing GitHub specific fields, including:
  • Title*
  • GitHub Repository URL*. Note: We will need to add a prefix here (similar to the about links section) that says: "https://github.com/" to make it clear what part of the GitHub URL the user needs to enter.
  • Ticket type* (Issue, Pull Request)
  • Ticket State*
  • Description*
  • New Column* (plus the supporting fields)
  • Community admin should have a way to preview GitHub board from the configure settings.
• If an admin enables the GitHub plugin, the GitHub field(s) with * above are required
• There is no disruption to old/existing GitHub plugins
• There is migration option provided for existing communities to transition to the new framework

Notes:
In order to edit the tab name issue #283 has to be finished implementing that functionality.

[Tguntenaar]

I'm closing this draft, it is already handled by PR 344