Add separate dark features to disable Bitbucket and GitHub sync independently

The current dark feature dvcs.connector.synchronization.disabled disables synchronization for both Bitbucket and GitHub.

We want to introduce three new dark features:

• dvcs.connector.synchronization.disabled.bitbucket
• dvcs.connector.synchronization.disabled.github
• dvcs.connector.synchronization.disabled.githube
  so that it works as followed:

  ...disabled	...disabled.bitbucket	...disabled.github	...disabled.githube	Bitbucket Sync Enabled	GitHub Sync Enabled	GitHub Ent Sync Enabled

Note: Make sure we update UI and controls to warn user and not allow them do things they are not supposed to do